Verified answer

If you go into matchmaking you can search through playlists like Arena and Invasion, in Arena you can play slayer and team slayer. Lone wolves is free-for-fall so you can play Slayer and Headhunter in those modes. It's not that you can't play those modes competively it's just that they have certain playlists in which the modes are interegrated in. Example: playing Arena will get you into a game that will give you the choices of Team Slayer, Team DMRs, etc.

its because xbox live is lame, nah ive only played xbox live for a couple of hours and dont really quite understand how it works, but from my understanding playing halo 3 you cant actually choose what map or game type you want, xbox live just links you up with a bunch of other similar skill level players and starts a game and it always seems to be boring slayer where you dies alot and cant kill other players.(maybe thats just cause im useless at halo, lol) maybe when you have a better ranking or something you can choose these things? im not sure, but i want to know the answer too. maybe you can only do that type of things if you invite lots of xbox live gamers that are in your friends list or something and host a game online? i dunno