Halo 3 counts nine levels, which complete the storyline of the Halo trilogy. The campaign can be played through alone, or with other players via Xbox Live.

Instead of having each player be an identical Spartan as in previous Halo games, the first player plays as Master Chief, the second player plays as the Arbiter.

The behaviour of enemies the player faces has been modified, giving them a "pack mentality" that causes the aliens to perform similar actions at the same time.

Halo 3 introduces a competitive aspect to cooperative play. Players are awarded points for defeating enemies while completing a level in the campaign.

Extra points are awarded for certain actions, such as performing headshots or defeating multiple enemies in rapid succession but are lost if the player dies or kills a team-mate.