It appears that a version of Gears of War 3 has leaked online. The game is not due out for a couple more months, on September 20, 2011.
According to NeoGaf:
"Heads up guys, a work in progress version of Gears of War 3 has been leaked. It apparently has the full single-player and all the multiplayer options.Buggy as hell, but it seems like it's all there. Avoid youtube and other forums if you don't want to be spoiled. There are videos all over youtube as proof. Just a heads up."
The work-in-progress version is said to be very buggy. So if you don't want the experience ruined for you, then just take their advise and be cautious of what you read and watch about the game.