“That is correct – up to four player co-op in Campaign mode on Xbox Live or System Link.” says Bungie’s Luke Smith in this post on Bungie.net
Like every other aspect of Halo 3, this mode is chosen from a Lobby. Simply select Campaign, choose your network (Xbox Live or System Link) and invite your friends to join you, or they can simply join your session once it’s set up and in a lobby (but they cannot join a game once it’s in-progress). They will pop into your lobby instantly, and when you have all the players you need (anywhere from one to four) you can begin the game from the beginning – or from any of your saved checkpoints.
Split-screen co-op is as ever, limited to two players per screen – but they can if they wish join two other friends online or via System Link.
For fictional reasons, we decided to avoid Halo’s slightly surreal compromise of two identical Master Chiefs and rather, use this opportunity to expand the Halo universe just a little bit. Player one will control the Master Chief, player two will control the Arbiter and players three and four will get the chance to control two brand new Elite characters.
Characters will be as follow:
Player One: Master Chief
Player Two: The Arbiter
Player Three: N’tho ‘Sraom (Elite)
Player Four: Usze ‘Taham (Elite).
So there you have it. 4 player co-op to finish the fight.