Hey OP. I had a post on the Halo subreddit around a year ago regarding this information, I'll paste the post here. Feel free to use any information from what I've gathered if you find it useful.
POST: Microsoft's language is "Deny. Deny. Deny until pestering is over. Confirm and release." The amount of time between each rule may vary. But most current Halo games were leaked but denied to hell, then they were released. 343 has seemed to be grabbing some rules from them, as they are very close when it comes to Halo. BUNGiE has no knowledge of Halo for PC though.
Halo CE, 2, and 3 were found in the Steam database. There is an official group on Steam for Halo CE, Halo 2, and Halo 3 something that not any user can make, VALVe has to. Clicking the group results in going to the Store and/or community page, depending on where you're trying to access it from. Halo 3 code was also found in the AMD drivers database. When CertainAffinity was asked about it on Twitter they replied with "We plead the fifth :)"
This can be found here: https://twitter.com/CertainAffinity/status/349235363996385280.
You may notice it has been classified as an "Unknown App", The db says:
"What is a "SteamDB Unknown App"?
This app has been deleted or is hidden to the public.
There is extremely limited to no information about this app.
We don't even have a name, hence why we are displaying SteamDB Unknown App "AppID".
Quote analysis:
"This app has been deleted or is hidden to the public."
"hidden to the public"
It was classified as an Unknown App 2 months ago as of the original submission.
Links to the Steam db pages:
Halo: Combat Evolved's ID is: 216760 (http://steamdb.info/app/216760/#section_history)
Halo 2's ID is: 216780 (http://steamdb.info/app/216780/#section_history)
Halo 3's ID is: 216800 (http://steamdb.info/app/216800/#section_history)
The AMD Driver code that caught people's attention was the following:
"<profile Area="UDX" Usage="Halo3" Notes="Halo3"> <value Property="Version">1</value> <value Property="MTShaderCompile">False</value> </profile>
<application Title="Halo3" File="halo3.exe"> <use Area="UDX">Halo3</use> </application> " *
Source: http://www.kn00tcn.net/site/halo-3-for-pc/
Here are 2 neogaf threads regarding this information: http://www.neogaf.com/forum/showthread.php?t=600471 http://www.neogaf.com/forum/showthread.php?t=525510
A video of Cool_Guy13 (KnifeonlyTV on YouTube) calling Microsoft Customer Support about Halo 3 for PC, the not-so-bright operator confirmed Halo 3 for PC without hesitation: http://www.youtube.com/watch?v=abb1InBtBmA
It has been reported (and I've heard witnesses) that certain games on GFWL have been relieved of the dependency of GFWL, this may mean Microsoft is slowly breaking down GFWL and moving to Steam. So Halo 1-3 for PC probably won't have a required GFWL account to play online.
(New evidence area starts here)
URGENT EVIDENCE FOUND, another representative has said the about same thing as the other video I sent, KnifeOnlyTV is in this video as well: http://www.youtube.com/watch?v=vrkrLOEjY8s&feature=c4-overview&list=UUtl22XrlA6hWAHS6vUxbRww
(Update as of 7-13-13) Recently, all Halo groups on steam now give an error saying: "Sorry! An error was encountered while processing your request:
There was an error retrieving information about the specified group.
Here's a link to the Steam Community home page."
If you think about it, before, you would be redirected to the community/store page (depending where you click it from), but now, it's letting you access it, but it gives an error.
(CRITICAL UPDATE: 8-10-13) More evidence recently from Certain Affinity's Twitter: http://i.imgur.com/LBwYQFu.jpg
(Update as of 8-22-13) These are mere theories but are in fact supported by evidence. The Games For Windows Live Marketplace has been shutdown, as well as the service itself in mid-2014, where could Halo 2 go? No other place but Steam, it will give Microsoft an incredible amount of money, due to the large amount of people that use Steam actively, it's a no-brainer. And here is something regarding the groups that makes sense, this is about the errors they give. There is a game in development called 'Overgrowth' that is going to be on Steam, it give the same exact error as the Halo groups do (as of writing this). Does this mean the group is being built? More than likely.
(9-7-13 update removed, merely theorizing)
(Update as of 9-11-13) Each of the Halo entries have been changed/updated, they have all been assigned a new changenumber, that number being "282126". The SteamDB FAQ says this about changenumbers/lists: "When an application or package changes, Steam sends out a "changelist" to notify all Steam clients that something changed. This changelist has a number, referred to as "changenumber". Do remember that you can see the links a little higher up the post, they were all updated at the same time, I'm not sure what this could mean, but it sure is something.
(Update as of 9-13-13) Certain Affinity just keeps on teasing us! When asked by another curious fan asked about when Halo 3 PC is coming, as he mentions he already knows it is, CA comically changes the subject with "Oh look...something shiny!".
(10-13 Update removed, contained nothing helpful to topic)
(Update as of 12-1-13) We may be onto something, but there isn't any concrete evidence involved so posting it here would just be mere speculation once more, if we find something or if something happens, it will go here right away.
The Amazon evidence isn't really valuable in our opinion, but it'll be still here, but italicized at the bottom of the post.
(Update as of 5-25-14) Halo: Combat Evolved, Halo 2, and Halo 3 have had updates in their SteamDB entries. It was earlier this month for each entry, but the change wasn't listed. Also, each sub has a billing type noted as "CD Key". See the links above for each game to see the sub listing and the absence of a noted change when a change was made.
(Update as of 6-9-14) So E3 happened, The Master Chief Collection was announced. It contains Halo 1-4 and access to the Halo 5 beta. It was mentioned to be exclusive to the XBOX One. Former GameSpot journalist Jeff Gerstmann said here in Giantbomb's E3 live stream he saw "move the mouse to move" on a Master Chief Collection demo, changing to "move the controller to move" when a controller was picked up. He then explains why "move the mouse to move" wouldn't make sense in a demo for a platform other than one you're developing for. It's now highly-likely that a PC version of The Master Chief Collection will come out sometime in the future or be announced for a PC version.
All EDITs are to fix formatting and to add more links.
(Urgent Update as of 12-23-13) We believe that retail Halo 3 for PC has just been confirmed. Someone I am working on the post with was chatting with Amazon support about another game, then he popped the question we have all been asking ourselves, which can be seen here, names have been blocked out for privacy reasons. He asks "Can you see if there is any information regarding a release of Halo 3 on the PC?" The support replied, saying that it is a product on Amazon, but there is no release date, which is shown in the picture.
Same-day update: We've got another person from amazon saying just about the same thing as the last one.
(Update as of 1-2-14) A fellow user by the name of gravity_bwlast commented with a picture of another confirmation of the existence of Halo 3 for PC.
(Minor update as of 2-27-14) Another contribution from the comment section of the possible existence of Halo 3 for PC. We'll be working on asking about the other 2 games as well.
