The Third Battalion was a unit of the UNSC Marine Corps. They were the present during the Battle of Camber, with their reserves stationed at the System Defense HQ. As the reserves were readying for battle, the building was infiltrated by Thel 'Vadamee and at least two other Sangheili. In a rare display of honor towards the humans, Thel allowed the marines to finish arming themselves before engaging in combat. The unit was annihilated by the end of the battle.