Checked This morning, took 2mins to find a game of Halo 3:
I was actually enjoying my game but had to go to work.
BTB from 10months ago:
Just some tips if really are interested nick, first off, the online is on dedicated servers, all 60fps/1080p. Secondly, Halo 2 and Halo 2 Anniversary are BULLET hard (for me anyway). Also, bare in mind in some cases you'll play 3 different versions of the same map, Halo2/3 versions of headlong, zanzibar, Halo3's Valhalla has 3 versions, almost 4 if you count blood gulch as a variant.
Campaign co-op is not on dedicated servers unfortunatley (it is on Halo 5) but I found Halo CE, 3 and 4 to work perfectly, I didnt play a lot of Halo 2's campaign cos its garbage. The online works like this, you can pick a game, say Halo 2 classic and it will cycle through 4v4 size maps with a mixture of slayer, CTF, Bomb and Stongholds (king of the hill). Same with Halo 3, 4, and even CE. If want to play Halo Doubles it will span across Halo 2 Ann, Halo 2 Classic, Halo 3 and Halo 4. BTB is also across all games.