Shared timeline

Background
This is a timeline based on the fact that Doom (2016) is a soft-reboot of the series and continues from Doom 3, but also Doom 64. . By soft-reboot instead of creating a new story from scratch  (throwing out all previous canon) it tells a somewhat unrelated story, while taking and building upon bits and pieces from the previous stories (compare Wolfenstein: The New Order and how it related to Wolfenstein (2009) for another form of 'soft-reboot').

The developers have never been clear on what is and isn't canon. Instead referring to pretty much everything including Easter eggs, the comic, etc, as "Doom lore" and history. They have never specifically indicated if Doom takes place on multiple and connected parallel earths, or a single earth (though Doom Eternal's own lore may suggest a 'single Earth' being fought over by Outside forces, including the same Hell, that Doom Slayer has beeing fighting for years across space and time). However, Art of DOOM Eternal does appear to suggest the idea of multiple universes (including Earths, although only he maybe aware of them).The fans can only speculate. (for timeline that largely separates Doom 3 and Doom (2016) as separate timelines see Doom 3 timeline & Doom 4 timeline).

For example there is lore even for the Player Customization;

In its analysis of this abberation, VEGA observes ephemeral distortions within the fabric of reality: scrambled reflections of the physical world. As the Slayer interacts with this anomaly, a peculiar ripple in the anomaly's composition emerges, seemingly reacting to the Slayer's presence reflexively. The anomoly reshapes itself, calibrating itself to the Slayer, responding to his memory, ancestral, and memetic data, before utimately projecting itself onto the Slayer himself. These change his appearance to many different suits (from those he wore back in his time during Doom 1, to his time in Quake Arena/Champions etc).

This seems to suggest that the single slayer has lived many lives, and possibly across many realities. He has memories of these past lives that can be accessed via anomalies and time travel. This may also explain his use of 'extra lives'. There maybe 'one earth' but actions of the Doom Slayer and Hell may have changed the direction of that history several times creating new 'timelines'. Flynn Taggart may be a version of him in one timelie, B.J. Blazkowicz a version from another, Marine from Doom3, yet another of his reincarnated lifetimes.

The 'classic-looking' weapons (based on Doom 1/2) the Slayer looks for in Doom Eternal, are described as the weapons he collected ages ago (and under Sentinel tradition) he honor-bound to find again, since it is way of the Sentinels to never lose their weapons. He lost the weapons when S. Hayden tethered him.

The guide also says Slayer has been fighting, a battle that continued for an eternity, a fight that can only be won by confronting his past if he hopes to stop history from repeating itself."

The dates on this timeline largely originate from Doom 3, Doom VR, Doom (2016), Doom Eternal, and Art of Doom books. Many of which do seem to fit into or share with Doom 3 dates (even if there is no direct references between each game stories: exception being The Hero/Doom Slayer possibly). A number of events are even referenced by Art of Doom/Doom 2016 that occurs on the same dates in Doom 3 (using similar descriptions/terminology). Several locations are also shared between games but generally only referenced Codexes such as Site 1 and Site 3.

As for Doom 1, 2, Final Doom, and 64. These are tied together into the backstory of the Doom Slayer/The Hero (as hinted in Doom 3, and reference to the Mother Demon). However it is not clear if those eras are from a 'alternate reality' or from the past of Doom (2016) universe. The only clue we have being that Doom Slayer has travelled Worlds and Time. To separate that one it is left to a smaller 'prologue' to this timeline at the beginning of this article.

This is not meant to be shared ID multiverse timeline so references to past events, Easter eggs, etc from other game franchises are generally left out (unless a date was given in the Doom games themselves), or any specific concrete references to their history. So very little of Blazkowicz family connection is likely  to make its way into this timeline.

It is not known if the SNES Doom timeline relates to this universe or not (or if it follows closer to retcons that place classic doom closer to the 22nd century (see Doom RPG, Doom II RPG, and Quake Champions) So direct dates for those games are generally ignored for this timeline. Novels and movies are also left off, as there is no indication of them being part of the same universe (Easter eggs to Flynn Taggart excluded), besides the fact that their dates usually do not line up with the games, and also take detours from the history set in the games. In comparison Doom 2016 actually fits into the framework of Doom 3's timeline and takes place four years after the events of Doom 3 with no direct contradicting elements (and contains several shared events).

Timeline
Note: From Doom Eternal's perspective the inhabitants of Hell, Urdak and Argent D'Nur, speak only of the "Earth Realm" in a singular fashion. As if they are only fighting over one version of Earth, not several and there is only one 'Hell' which has been plaguing Earth with its monstrosities (implying that all previous Doom games had to exist within one shared universe). The Ancient Martians of Doom 3, are in fact the Hebethen Sentinel civilization of Mars. Art of Doom Eternal introduces the idea that there maybe more than one parallel Earth, and Doom Slayer has memories, ancestral and memitic to these universes. Thus original Doom universes he once belonged in isn't necessarily on the same timeline as Doom (2016) but could exist in another universe. Also time travel might have 'changed' events creating parallel earth histories as explained in the multiverse theory..

Prologue
21st or 22nd century
 * Doom 64 ends: The Doom Marine enters hell one more time and vows to fight demons for eternity to prevent them from invading earth again.


 * The Lost Levels: The sister Resurrector decides to take revenge on the Marine, sending him back to Earth. He fights his way through a UAC base to find a portal back to Hell, fighting his way through the new patch of Hell he finds the Mother Demon's sister again, and takes takes revenge on her. After her defeat he again decides to remain in hell walking the perpetual path through Doom.


 * He is discovered injured by the Night Sentinels who nurse him back to health and make him their Champion.
 * Worlds and Time
 * Doom Eternal flashbacks

Ages
Note: Exact order of these is not known.
 * Golden Age
 * Metal Age
 * Time of Grief
 * Infernal Age
 * First age
 * Second age
 * Third age
 * Fourth age - This appears to be the era in which Doom 2016 takes place.

Known History
(Doom 3, Resurrection of Evil, The Lost Mission, Doom Resurrection, Doom (2016), Doom VR, Doom Eternal, Art of Doom series)
 * 80 million years ago.
 * 60 million years ago.
 * Millions of years ago.
 * Doomhunters are frozen in the Arctic and preserved there below the frozen ice.
 * c. 10,000 years ago.
 * 149 (AE/:1366/88)- First recorded trans-dimensional encounters with demons and humans apparently on Earth. First time demonic races were documented by humans (this may also be associated with Argent D'nur or the Bethians).First known transdimensional encounters occured over two millenia ago.
 * 1996 - The first Quakecon Games is held.
 * c. 2019 (assumed date)
 * Samuel Hayden is born.
 * Early 21st Century; Water is discovered on Mars. Very little happens in way of colonization, except exploration over the next century.
 * Weapons from this period are proven, and still used into the 22nd century.
 * 2049
 * Plans for the colonization of Mars begins. UAC, Mixom and I/O Logistics lay down the foundation of a new civilization on Mars.
 * 2095
 * Late 21st Century; Advanced Rocket Launcher is developed.
 * October 29 - Construction of Phase One, Alpha Labs begins on Mars.
 * The Argent Fracture is discovered. Argent plasma was discovered in a trench on Mars by an expedition from the SS Amundsen.
 * 2096
 * The Union Aerospace Corporation established an outpost on Mars to extract Argent plasma from the Argent Fracture.
 * 2100
 * January 1
 * Early tethering operations firsthand discover possibility of species-demon hierarchy.
 * 2104 - The explorations of the caverns in 2104 led to the discovery of ancient artifact codenamed U1, the Soul Cube, along with stone tablets written by the ancient Martian civilization.
 * 2115
 * Martian Civilization was first discovered.
 * A region known as the Well is discovered on Mars.
 * 2117 - Argent facility construction.
 * 2118 - Steve Jenson is born.
 * 2120 - Construction on Mars City begins.
 * 2122
 * March 10 - A statue of the Praetor Suit was excavated (Artifact U9) from Site 3.
 * 2124
 * November 16 - Two Ancient Sentinel statues were excavated (Artifact J46) from Site 1.
 * 2127
 * The Argent Tower is completed.
 * 6 Months Later - Samuel Hayden transfers his mind over to a cyborg body to continue his existence, after his body breaks down due to Stage 4 Cancer.
 * 2130
 * July 17 - The Alpha Labs are completed.
 * 2136
 * The Advanced Research Complex is completed.
 * Advanced Weaponry Division is completed.
 * '2139 - Project Mercury, built in Delta Labs, becomes operational.
 * 2141 -
 * January 15 - Alpha Labs Introduction video is made.
 * K. Tang is transferred to Mars.
 * 2142
 * Mach-2 Chain Gun Briefing video is produced.
 * 2143
 * During one of the first teleporter explorations, the Corrax tablets are retrieved from Hell in a UAC Automated Survey.
 * 2144
 * Early manned UAC Expedition to Hell.
 * January 1 - Research on a partial Imp remains.
 * February 5 - Hell Knight (partial) research.
 * July 28 - Series 3 Plasma Gun information video is released.
 * November 2 - A Living Dead is recovered and researched.
 * 2145
 * March 5 - Mars Research Facility One wecome video is released by Human Resources.
 * March 11 - Marine Command Briefing video is created for incoming transfers.
 * April 5 - UAC engineer Samuel Harding completed repairs on the Monorail.
 * April 15 - Hydrocon PR video is produced. Molecular Fuel Storage Compactor PR vide is produced.
 * May 8  - The first Project Lazarus Manned Expedition is sent to Hell. The expedition ends with complete loss of human life, but achieved its mission. The sarcophagus of the Doom Marine is recovered from Kadingir Sanctum.
 * June - 21:00 hours UMT. Experiment in the Delta Labs triggers an unknown event, resulting in the systematic invasion of the entire facility. Marine forces are deployed to hot spots around the facility, with many teams encountering heavy resistance. Bravo Team was ambushed in EnPro facility while searching for survivors. There was only one survivor of the team.
 * July 12 - A Imp is recovered and researched.
 * August 6 - Simon Garlick is assigned to Expedition 14A-2. Heather Elaine Earl Besch
 * August 8
 * Simon Garlick "misplaced" the rest of Expedition 14A-2 after entering into Hell.
 * Archaeologist Richard Davis discover a compelling discovery from the Site 3 ruins that the original inhabitants of Mars evacuated to Earth.
 * August 10 - Simon Garlick discovers that his team are killed and mutilated by demons. Richard Davis reports his findings on Site 3 to Malcolm Betruger.
 * August 21 - Ancient Civilization video is produced. AM3 Series Portable Power Plant video is produced.
 * September 5 - Charles Hollies, director general of the EAP, announces that all environment processors be brought offline in the general area of Site 2.
 * September 8 - Pierce Rogers
 * September 14 - Andrew Chin records a complaint regarding to UAC Security harassing his research staff in the Alpha Labs.
 * September 19 - Frank Cerano thinks on Malcolm Betruger's ideas for sustaining teleportation portals.
 * September 23
 * The UAC Board unanimously approved a resolution for a full operations review of the Mars Research Facility at the bequest of Elliot Swann.
 * Charlie Haskell reports progress in Chamber 3 in the Delta Labs.
 * September 24 - James Holiday, a transport controller, reports recent transport issues concerning better ways to provide protection for Site 3 artifacts.
 * September 25 - Tony Bates, tired to go through the code for the door systems Central Processing, issue a backdoor code into the systems.
 * September 27 - Revenant research
 * October 1
 * Nick Sadowayj, a HAZMAT specialist, conclude that the Martian atmosphere is wreaking havoc on the exhaust valve seals in the standard Number 5 disposal drums.
 * A order of DragGonFeets action figures is accidentally shipped to the Exis Labs. The action figures are accidentally broke by Joel Stangl.
 * October 2
 * Malcolm Betruger writes to William Banks that cargo bearing religious artifacts arriving from earth are to be directly sent to the Delta Labs.
 * Elizabeth McNeil writes to Elliot Swann that she is very pleased of his request to investigate the UAC on Mars and her subsequent reassignment from Mars to Earth.
 * October 3
 * Malcolm Betruger attempt to sway Elliot Swann from conducting a full operations review on Mars City.
 * Bernice Tooley writes to Harold Munch about a employee toying with a Grabber gun in Exis Central Operations.
 * October 4 - Harold Munch writes to Bernice Tooley on having caught technicians toying with the extra chainsaws in Communications Transfer.
 * October 5
 * William Banks reports the recent power grid changes have caused many non-critical systems to malfunction, all of which is due to Malcolm Betruger's 'optimizations' to the energy stores in and around the facility.
 * Maintenance Safety Video is produced, Specimen Stasis Museum video is produced, Soul Cube video is produced. Tablets video is produced.
 * October 6
 * Central Security Authority writes to Karl Cullen concerning a automated security turret gun will be installed at the main entrance to Delta Lab due to the increased sensitive nature of the experiments and security problems.
 * Dmitry Lebedev received the latest shipment of the MK7 space suits.
 * October 7 - Ben Wolfe reports of "unauthorized transmissions" uncovered in the off-site redundant logs.
 * October 8 - Chase Higgins spends 20 minutes calming his co-worker Morgan, who sworn to have seen a huge bloody eyeball stare at him through the glass shielding in the Particle Exchanger.
 * October 9
 * Chase Higgins writes to Nasir Adil about the strange occurrences surrounding the Particle Exchanger.
 * October 10 - BFG 9000 Briefing video is produced.
 * October 11 - Martianbuddy.com
 * October 12 - Chief Engineer Allen Rhodes reports significant progress in Energy Processing Sector One.
 * October 13 - The first violent incident at the Exis surface tunnel. One of the construction workers, Bill Sanders, was offloading a shipment of steel supports when a procurement manager stopped him. In the heated conversation that followed, Sanders grabbed a nearby pickaxe and lunged at the manager. Luckily the argument beforehand generated a sizable crowd, and when Sanders attacked the manager, a number of men were able to subdue him.
 * October 15
 * Frank Cinders describe his first experience of surviving through Hell.
 * Tony Bates resume his investigation into case A-10982.
 * October 16 - Armor Corp, First Platoon and First Science Team are completely wiped out on their second excursion into Hell. Han Lee is in charge of writing up their reports and sending this information back to their next of kin on Earth.
 * October 17 - Pierce Rogers
 * October 18
 * Frank Delahue reports the constant and increasing load from the Delta Complex that is causing havoc on Energy Processing.
 * Robert Cody reports in his audio log that working in the caverns underneath Delta are too dangerous and regards to hearing "howlers".
 * Steve Bryant writes to Dmitry Lebedev regarding to the tight security in the Exis Labs.
 * October 19
 * Grant Baston record on a alarming number of reports of unexplainable incidents in the Mars City Underground.
 * Larry Bullman notes that he is coming closer to decipher the glyphs on the Soul Cube and confirming the possibility that the artifact is a weapon.
 * October 20
 * William Banks reports on a alarming number of missing personnel.
 * Phil Wilson witnessed the third test through the teleporter in three weeks.
 * Han Lee report in his audio log that another group of researchers were sent to Hell two days ago and they failed to return at their scheduled time.
 * October 22 - Andy Cavazos report to construction supervisor Travis Folsom about a series of very unusual seismic activity (which should not exist on Mars) that collapsed a number of foundation piers at Exis Research.
 * October 23 - Gary Ross sends a reminder to Delta Security that the new Automated Defense System, at the Delta Labs entrance, is fully operational.
 * October 24
 * Eric Nelson writes a report to Bernice Tooley on the delays complicating Exis Labs. Reclamation problems continue to plague the main processor in Energy Processing. Jack Smith makes his report on the number of accidents in Alpha Labs.
 * Henry Nelson makes a request that a security team make a thorough check of the EFR area after reports of ghostly voices are heard in the area.
 * October 25 - Mathew Morton suggest to Steve Hammer of locking up the weapon lockers after the violent suicide of Private Swensen.
 * October 26 - Henry Nelson writes to Michael Abrams concerning on the reports of strange noises heard in the Alpha Labs that are frightening his staff and request a security detachment to do a sweep of the area in Alpha Sector 4.
 * October 27
 * Chris Vargas changed the security code of his display cabinet per accordance to Central Security.
 * Andrew Chin announce preparation for Football Sunday.
 * October 29 - Peter Raleigh report that there has been no success to treat the Mars personnel of their dementia and finding no evidence for its cause.
 * October 30 - Staff Sergeant Bill Tyson announce of new troops that will be arriving to Mars and doubling security. Anthony Garza, Paul Simons
 * October 31 - Halloween is celebrated on Mars City.
 * November 1
 * Start of Safety Awareness month.
 * Mark Lamia files a complaint on the incompetency of his staff in the Alpha Labs.
 * November 2 - Allen Rhodes, Travis Folsom, Steve Jenson medical report.
 * November 3
 * Theresa Chasar reports that the ammo storage in the Mach-3 Plasma Gun has far exceeded her and her research team's expectations.
 * Nasir Adil reports of increasing level of random and unexplained violence in the Exis Labs.
 * November 4 - Jeffery Moen
 * November 5 - Chris Vargas reports on the initial testing on the primary alignment module indicated excessive stress fracturing. Vargas immediately discontinued the tests until the module is repaired.
 * November 6
 * Jenny Lin broke into a supply cabinet containing emergency weaponry supplied to the local security teams. For reasons unknown she then became hostile and killed two security officers. Another security officer was injured while bravely putting himself in harms way. Technician Lin was also killed.
 * November 8 - Bradley Martin prepares a message to the Service Sub-Level Staff regarding to the Jenny Lin incident.
 * November 9
 * Dr. Marcus Caseon sends his audio reports to Earth regarding to an assault on his life by a disturbed patient.
 * Bradley Martin announce to the Service Sub-Level Staff that a full investigation of the Jenny Lin incident is currently under way.
 * November 10
 * Maintenance technician Adam Berneche hears strange whispering that are nowhere to be found.
 * Robert Thompson complains about a potential cover up in the Jenny Lin incident.
 * November 11 - Scott Johnson, a pump specialist, writes to a fellow employee about a unknown creature that was reportedly seen in Sector 2 of the Recycling Plant.
 * November 12
 * David Robbins of Industrial Complex writes to Marcus Stanton regarding to a shipment error of several crates of chainsaws.
 * Nasir Adil received word from Central Authority about the upcoming visit of Counselor Elliot Swann. He also notes that due to the violent assaults in the Exis Labs the civilian staff have grown wary of one another. Furthermore, weapons are turning up missing in the security checkpoints.
 * November 13
 * Administrator Leslie Waters writes to Phil Anders that all storage cabinets are given limit access to senior level staff in order prevent a Jenny Lin incident.
 * Elliot Swann writes to Nasir Adil and heed his concerns on the Exis Labs.
 * November 14
 * Ian McCormick file a report on his studies on the Soul Cube in which he discover its use as a weapon "to repel whatever type of creatures" that invaded ancient Mars.
 * Ray Gerhardt notifies Duncan Mathews that he is leaving Mars and UAC. Robert Finch, Data-File AR-U1
 * November 15
 * The Doom Marine arrives on Mars on Transport 4409 at 6:18 MC, he was transferred as punishment for assaulting a superior officer for ordering his soldiers to fire upon civilians, and to replace another soldier. He and his body cast were shipped to Pearl Harbor, while the Marine were transferred to Mars, home of the Union Aerospace Corporation.
 * DarkStar is scheduled to return to Earth destination Burns Flat, OK at 18:55 MST.
 * Engineering manager Frank Delahue writes to Daniel Young that he is considering on a promotion for him and replace a engineer in Reclamation.
 * Duncan Mathews request weapon and armor from UAC Security due to his concerns on the strange incidents in Mars City.
 * Ian McCormick records in his video disk on human teleportation experiments.
 * Richard Meyers (Teleportaiton Experiments video)
 * Allen Rhodes and his team barricade themselves in a control room in Sector One.
 * +6 hours post invasion, Bravo Team is ambushed, Bravo Marine (ID# 6703x-02) is the only survivor.
 * November 16 - Elliot Swann, Ron Ridge, Valuable Sacrificial Pointers
 * November 20 - The Recon Zulu marine team found only one survivor, a Marine Corporal. Important UAC personnel such the company's lawyer and counselor Elliot Swann and his bodyguard Jack Campbell, who arrived hours before the outbreak to make an assessment of the whole Mars operation and enact damage control if necessary, were found dead. Master Sergeant Thomas Kelly and other Delta Labs scientists were also found dead. The marine team had no information regarding the fate of Dr. Malcolm Betruger, the chief scientist of Delta Labs who - unknown to them - had become the demon Maledict.
 * November 21 - Ionized Plasma Levitator Briefing video is produced.
 * November 22
 * Expected arrival of T4315 at 06:00 MST from Lunar 1.
 * November 26
 * Expected arrival of F36 at 14:30 MST.
 * November 29
 * Expected arrival of T4500 at 06:00 MST.
 * December 6
 * Expected arrival of T4800 at 06:00 MST.
 * December 12 - Sturton docked vacation time of anyone caught playing too much Super Turbo Turkey Puncher 3.
 * DarkStar scheduled to make it to earth 13:00 CST.
 * 2146
 * Industrial accident related deaths in the Foundry dropped significantly thanks to new UAC safety protocols where employees exposed to dangerous materials are sent to the Lazarus Labs.
 * May 1 - An orbital probe detects a faint unidentified signal broadcast from the abandoned Site 1 Complex.
 * May 28
 * Twins demon is discovered, and researched.
 * August 6 - Mars mission Goals & Safety video is produced.
 * August 8 - The UAC announces the renewal of its Mars research program. Dr. Elizabeth McNeil is chosen to lead the primary assessment team.
 * September 12 - Elizabeth McNeil welcomes personnel to the Site 1 Expedition.
 * November 27: UAC Lead Technology Officer releases the Ionized Plasma Levitator Briefing video.
 * December 17 
 * The first Project Lazarus Tethering Operation is conducted.
 * A 'new' breed of Pinkies are discovered and researched.
 * The "vade mecum" compendium is recovered from the Umbral Plains containing information on the Aranea Imperatrix.
 * 2147
 * Central Senate projected to cut deficit in half by this year.
 * January 6 - Bernard Fresko, Roberts
 * January 8 - Bernard Fresko
 * January 10 - Bernard Fresko
 * January 12 - Bernard Fresko
 * January 16
 * The second Project Lazarus Tethering Operation is conducted. The expedition ends in an confrontation with the Hell Guard.
 * They recover a hell warden.
 * March 10 - Ron Gibbons, Nathan Reynolds, Ronn Jolson, Viktor Kharkov
 * March 11 - Preston Glenn, Kleinberg, Mac Smith, James Owens, Chance Glasco, Nicolas Haag
 * March 12 - Ryan Okeefe, Ian, Preston Glenn, Stocker, David Edens
 * March 13 - Marcus Tanner, Viktor Kharkov, Gregory Martin, Elizabeth McNeil, Preston Glenn, Fred Grossman, James Owens, Elizbeth McNeil, Keith Noordzy, David Edens, Nicolas Haag, Kelly Garland
 * March 14 - Marcus Tanner, Jack Martin, Sean Weston, Gregory Martin, Stewart Jackson, Viktor Kharkov, Steven Crane, Elizabeth McNeil, Ron Gibbons, Evans, Sean Gibson, Sparks, James Carnagey, Fred Grosman, Keith Noordzy, Ronn Jolson, Dr. Cloud, Patrick Hook, Mitchell.
 * March 15 - Sean Weston, Gregory Martin, Viktor Kharkov, Stewart Jackson, Jake Hernandez, Steven Crane, Leo Avery, James Carnagey, Elizabeth McNeil, Sparks
 * March 16 - Stewart Jackson, Steven Crane
 * March 17, 2147
 * Resurrection of Evil begins.
 * Jake Hernandez
 * 2148
 * May 19 - Doomed UAC Trans-Dimensional Expedition most of the explorers were set upon by Barons of Hell.
 * June 26
 * The second Project Lazarus Manned Expedition is sent to the Great Steppe. The expedition ends with similar losses. The petrified remains of a Baalgar demon is recovered.
 * 2149
 * c. February - The Argent Facility - UAC base had its last accident.Echo/hologram UAC facility level, would be accident free for 221 days
 * October - Doom 2016 Begins.Oct 2149 listed on first computer terminal in the game just after the Doom Marine escape from the sarcophagus.
 * December - Indentured servants on the offworld Tei Tenga Mining Station were replaced with robotic workers, optimal output expected to return to normal by this date.In Foundry there is a news terminal referring to 2149 'fire dust production estimates' on Tei Tenga Mining Station' in an offworld report 49
 * 2150
 * 2151
 * 2163
 * Doom Eternal begins.

Behind the scenes
The basic layout of the timeline in Doom (2016)/Eternal is almost exactly the same as that in Doom 3 (and roughly the same as in Doom RPG).

Man of the backstory events mentioned in Doom 2016 overlap to very similar equivalent events, if not the same events as mentioned in Doom 3 (and Doom RPG). All three take place in the 22nd century.

Chronologically speaking Doom 2016 takes place 4 years after Doom 3 (and Doom RPG). Which could allow plenty of time for UAC to rebuild and restructure since the events of Doom 3. Also Mars City was a huge place in Doom 3. Not all of it was explored, and it too three or four different campaigns to explore portions of it, counting Doom RPG as well. Nor did we see any other potential landmarks/populated centers on the planet (though maps in Doom 3 could have hinted at amny others). This could suggest that the base in Doom (2016) is just one of many of these various UAC bases scattered across the planet (one devoted to more occult studies, and requiring the initiates to join the cult), as opposed to more mundate industrial/irreligious institution that existed in other bases on the planet.

Furthermore there are more direct connections seen in Art of Doom (and potentially in Art of Doom Eternal) discussed in the back lore of the book.For example a reference to the region known as the Well being discovered in 2115.This appears to be taken from dates in Doom 3 ( The date when the Martian Civilization was first discovered).

While ignoring the influence of time travel (Worlds and Time and references in Quake Champions) it might be safe to see Doom 3 Marine) as obviously different character than that of Doom Slayer (despite hints given in Quake Champions and Doom RPG's website). As its one of the more confusing aspects of this timeline as it would indicate that if they are the same person they 'co-exist' roughly in the same time frame.

Other little connections are made Doom Slayer appears to be an interpretation of the The Hero discussed in Site 3 in Doom 3. The Praetor Suit statue (Artifact U9) is part of the U-series Artifacts the same collection that Soul Cube was part of (Artifact U1). Some of the locations/backstory for the Night Sentinels mirror that of the ancient Martians (discussed in Doom 3). They even share similar names Praeleanthor and Praetor. A number of other details pointing to the Soul Cube can be seen if one looks closely within the environment througout the levels. All seem to indicate the archaelogical studies connected to the The Hero, relate to the same being known as Doom Slayer. If one looks closely these details are shown in Doom 2016 as well.

In Doom 3 there is discussion of Mixom and Moxim lawsuit and a threatening to merge the companies into one in the future... In Doom 2016 we have Mixxom corporation which is exactly that a the merged company. There are other little details if one looks at environments closely that tie details from Doom 2016/Doom Eternal back to Doom 3 universe.

Even minor details like the shared appearance between some monsters such as Hell Knights in Doom 3 and Doom 4/Eternal, and lesser extent the way the Imps fight in Doom 3 in both Doom 3/Eternal. Or the unsued wherehouse containing Turbo Turkey Puncher 3, also a game seen in Doom 3.

Developers have hinted this may all be intentional too, coming short of saying specifically if Doom 3 is or isn't part of any seperate universe. In fact some of the articles at Slayer's Club seems to link many of the 'secrets' or calling them 'series lore' for the current series.

The Art of Doom Eternal establishes that UAC originaly wasn't cult-like business.. But that UAC splintered into competing factions. It was one faction that arose out of the Lazarus facility... The first cultists arose out of the UAC reponsible for the study and experimentation into Hell, in time drew them to its power (much as was the case with Bertruger), and choosing to open a pathway to evil. They formed a secret society founded in the occult, quickly mutated into cult of ritual sacrifice and blood offerings to ancient demonic gods. Over time they were corrupted by Hell's influence, became united and broke away from the UAC, establighing their own presence on earth. This at least indicates that not all of UAC were cultists like those seen in Lazarus and Argent Facilities in Doom (2016), and that earlier than that they were regular scientists much like those seen in Doom 3.

One Earth Dimension or many
While Doom Eternal currenlty only refers to one Earth dimension. Some other ID software games such as Quake Arena, and Wolfenstein Young Blood do establish there is multiple alternate earths. But it's not clear if this is the case with Doom universe yet (other than if Quake Champions is taken into account. Stil Doom Eternals plot does seem to imply that the demons are only really specific in one particular Earth realm. and it might be the same one seen in every game of the Doom series (Doom Slayer himself has not bee confirmed to be from an alternate Earth as of yet, bu tonly having been from Earth).. Still it's not been confirmed where and how or what place in the timeline the developers see the events of Classic Doom/Doom 64 games, other than it is part of Doom Slayer's past.