3 Player Local Co-op

Halo CE and Halo 2 only allow 2 player co op, unless you mod it you will not get 3 players. This glitch will only work in Halo 3.

Halo 3 shipped with the ability to play local with 2 players on via Xbox Live with 4 players on at least 2 Xbox 360s. However, it is possible to log a person in to the game (on Local) just before the game starts.

First of all you will need to log in 2 profiles on Local, they can be temporary profiles or Gamertags. Select your level and set up the game as normal. Get controller 3 ready but don't login. Next, using Controller 1 or 2, start the game. Then, while it counts down, press A on Controller 3, select Temporary Profile. Wait for the screen to fade, you have to select Yes just as the screen is completely black. If you select it too soon the screen with return to the lobby and the count down will stop. If this happens log Player 3 out of the lobby and try again.

[YouTube Tutorial]