3 Player Local Co op

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.

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 any 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.

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. This is likely because of the effect used in order to make Cortana appear on the screen, as Gravemind Moments only change the player's field of view and overlays a screen effect, much like the recording on Jenkins' helmet in 343 Guilty Spark. The two effects are very much different in that one is animated and the other is an overlay, although both play a sound clip and slow the player down.

Because of this, the best levels to use this glitch on are:
 * Sierra 117 Rally Point Alpha
 * Tsavo Highway Mission Start
 * The Ark Mission Start
 * Halo Mission Start.

Cause
This happens because the third and fourth players are able to join during the countdown. When they join while the countdown is initiated, the level starts before the game realizes there are too many people in the game.