I dunno.
I played the MCC front to back about 2 months ago, and honestly I thought Halo 3 was a pretty jarring come down from Halo 2A. You're right in that 60/1080 was still a nice upgrade for Halo 3, but I thought 2A was absolutely gorgeous - easily my favourite time with the collection - and I think Halo 3 looks rough going into it immediately after 2A.
Not sure I'd be lining up to pay full price for it but.. I think Halo 3A would still be something pretty cool to have.
I'm gonna have to agree with the other guy. I wouldn't be interested as much in a H3A. Not only because unlike the other two games, Halo 3 still looked decent enough because it was a gen ahead, but because how many times will people buy into it, especially after MCC. I mean the game went free on Xbox 360 once, it's now available on Xbox One in 1080p/60fps as part of MCC, as is the entire multi-player suite (whatever playable state it is).
I can start up Halo 3 right now, any level, any map, with 1080p/60fps. You couldn't do that for Halo 2 on Xbox One. Halo CE, it was their first real release/project after getting the Halo rights and was the first of the ten year anniversary for the series. They also utilized it to try and bring players back to Reach with the map pack.
Will I really want to buy Halo 3 Anniversary for improved campaign visuals? Pretty obvious they won't be able to have the same instant switch feature without a massive sacrifice to whatever H3A would run at, just look how much trouble Xbox One had with Halo 2.
Then comes multi-player. I loved the hell out of Halo 3 multi-player. But a large portion also hate it. What could they do to incentivize the multi-player? Maybe a full remake of ALL Halo 3 maps with Halo 5 gameplay? But then, would they really do that? Especially when the goal 2 years from now is probably Halo 6? One of the hardest aspects is adapting the maps of Halo 3 to the new features of Halo 5 gameplay. Maybe they could do ten maps? And a beta of Halo 6 to try and sell the game?