Spoilers and unseen gameplay are rife in the leak and the studio is working with internal "security teams and law enforcement". It retails September.
Epic's latest Gears of War joins a growing string of high profile leaks this year alone, which include a near complete PC build of Crysis 2 and PS3 exclusive Killzone 3.
"We have seen the reports of Gears of War 3 content being propped on the Web and working closely with our security teams and law enforcement to address the situation," said Epic Games in a statement.
"This content is not from a final build and is not representative of what fans will enjoy when the game launches worldwide on 20th September 2011." Ironically the leaked PC Crysis 2 may have led to a boost in pre-order sales, some speculated at the time.
Gears of War 3 releases exclusively on Xbox 360 September 9th. A leaked press preview build of Deus Ex: Human Revolution has been 'embraced' by Eidos - what a world.