There has been a long held rumor that Halo 3 will release on Steam- this actually goes back to some leaks that came via a Steam Database file dump some time ago. There has also simultaneously been the rumor – more like hope, really, but still – that Halo 3 Anniversary, a remaster of the classic capper to the original Master Chief trilogy, will be released this year, with it being the tenth anniversary of Halo 3‘s release.
However, 343 Industries’ community manager Brian Jarrard, has decided to break your heart by shooting down these rumors once and for all. Speaking on Reddit (via WCCFTech), Jarrard said, “I’ve been here just about 6 months now and haven’t ever heard of one single mention of Halo 3 coming to Steam nor Halo 3 anniversary being made. In fact, it’s been completely debunked multiple times over. Sorry to say but there’s no plans nor any work underway to make this happen. (though personally I can’t deny it would be sweet).”
So there you have it, folks- for now, if you want to play Halo 3, the Xbox 360 version is your only bet (though at least you can play it on Xbox One via Backward Compatibility).