Gears of War 3 takes place 18 months after the end of the second game, and picks up on the on-going story from the Gears of War books, the last one being Coalition's End.

After sinking Jacinto in order to flood the Locust Hordes' subterranean homeland, the Coalition of Ordered Governments (COG) was forced to relocate their entire population to their former island stronghold and navy base on Vectes via Port Farrell. The growing threat from the emerging Lambent, however, has now forced the COG to abandon Vectes Island as well. As a result, the COG has been disbanded, and its population split up into smaller communities to avoid drawing attention from the Lambent. The player will once again assume the role of Sergeant Marcus Fenix, leader of Delta Squad, currently aboard the flotilla of ships housing the survivors as they deal with the relocation and fight off the lambent threat.

Gears 3 retains the shooter gameplay used in previous games, using over-the-shoulder third person system, as well as a cover system that allows players to move from cover point to cover point, roll and blind fire weapons/lob grenades. Players are now able to knock down opponents as a result of vaulting over short cover points. Players can also do a 'roadie run', which is essentially a sprint, except in a crouched stance to reduce profile. Also making a reappearance is the active reload, which speeds up reloading if the player hits reload again quickly enough, or a damage boost and a quick reload if the player times it to perfection.

New to the Gears series is the ability to play with four other player's over Xbox Live, with the three other players taking on the role of Delta Squad. Gears of War 2's five player co-op Horde Mode also gets a revamp to Horde Mode 2.0. Also included is the new Beast Mode, the opposite of Horde, in which players fill the shoes of a member of the Locust Horde attempting to survive wave after wave of COG soldiers. In addition to these, all previous multiplayer game modes return.