Looking at Halo 5: Forge on PC, it's far from thriving, with around 20-60 players online. On top of that you can pretty much play Halo 3 multiplayer on PC already, and while that usually got some more players, it's still far from thriving.You can't compare a forge only mechanic to a probable community of a whole series. I know a load of people who play source games like counter strike with me, and when we talk about old games like the halo series, everybody has something to say about it. Nobody wants to touch just a forge onset, and I personally want nothing to do with the new Halo games or the way 343 are taking them. I'm talking about the best selling Halo ever made, not some garbage grab thrown together by 343. You can't compare a forge mode to the best selling Halo of all time lol.
If they want the game to be successful on PC, they need to make sure they market it for the PC community, and not just the Halo community, like they mostly did for Halo 5: Forge. Sure, there are a couple of PC players in the Halo community, but at this point it's pretty much just a console community since the newer games didn't released on PC for the longest time.
What I mean is that Halo 3 on PC can thrive, and it very likely will if they make sure to market it properly. But if they only release it on the Windows 10 store and don't inform the PC community about it, then it will likely fall flat. Because at this point, the PC community as a whole have moved on from Halo, so they got to make sure that people know it is back.
As for Halo 5 on PC, it's not just forge, it includes full custom game support as well as a way to play with other people. And while you might not care about it, looking at the player count for it, clearly indicates that Halo isn't popular in the PC community. I mean, there are on average 40 people playing the PC version, compared to somewhere around 20.000 people playing it on Xbox. Sure, the PC version doesn't include the entire game, but there's definitely enough content, so if the PC community cared about Halo, you would expect far more players than that.
And as I mentioned, you can basically already play Halo 3 multiplayer on PC. Sure, it's not an official version of Halo 3, but it is still using the Halo 3 engine, with Halo 3 maps, weapons and vehicles that functions like they did in Halo 3. And in terms of the player count it's about the same as Halo 5 on PC.
Sure, Halo 3 on PC would likely be a lot more popular, but I doubt it would literally get 1000 times more popular (40.000 players) than Halo 5 on PC without any serious effort put into the marketing.