3 Player Local Co op

From Halopedia, the Halo Wiki

The 3 Player Local Co-op glitch allows players to bypass the two-player limit when playing Co-op in the Halo games. In Halo: Combat Evolved and Halo 2, this is done strictly through modding; only in Halo 3 is it possible with a glitch.

[edit] Instructions

Halo 3 is shipped with the ability to launch two-player Co-op mode on Local Play; or with four players over Xbox Live or System Link, using at least two Xbox 360s. However, it is possible to log a person in to a Local Play game just before the game starts.

First of all, you will need to log in to two profiles on Local Play; they can be either temporary profiles or gamertags signed into Xbox LIVE. Select your level and set up the game as a Normal difficulty. Get Controller 3 ready, but don't log in with it.

Using Controller 1 or 2, start the game. As the timer counts down, press A on Controller 3, and select Temporary Profile. Wait for the screen to fade, and select Yes just as the screen goes completely black. If you select it too soon, the screen with return to the lobby and the count down will stop; to remedy this and reattempt the glitch, sign Player 3 out of the lobby and sign them back in, bearing the instructions in mind.

[edit] Cortana Moments

Please note that when using this glitch on a level containing Cortana Moments, the game will freeze when the Moment occurs. Oddly, though, Gravemind Moments play just fine.

Because of this, the glitch can only be used on Tsavo Highway, The Ark, and Halo.