You have it completely wrong, it's just that all the games are together and under a single UI with matchmaking mixing them together. If you play a Halo 3 map you will be playing Halo 3, not Halo 4 on a Halo 3 map.
Oh....man, that's actually really dissapointing. So no way to play Halo 3 on a Halo 4 or 2 map?
I'm really kinda bummed now.