Crash Site was also featured in Crysis 2 and, for all intents and purposes, is king of the hill. Two teams - 6v6 on console, 8v8 on PC - chase down and defend "pods," accruing points for standing near one. After a certain amount of time, the pod explodes, another is deployed elsewhere on the map and the cycle repeats. The pods themselves house two portable shields that aid in defense, and there's also the Pinger, a massive controllable mech that can help in either assaulting or defending the pod. Beyond that, however, the mode is pretty standard.
Far more interesting is the Hunter mode, which should be familiar to any FPS player that has ever played a game of "zombies."
Gallery: Crysis 3 (5/16/12)