Epic Games VP Mark Rein said in an e-mail, "Microsoft [the game's publisher] confirmed to me this afternoon that Gears of War 2 sell-through surpassed 3 million units as of last week."
2006's original Gear of War took around ten weeks to hit the same mark. Gears 2 did it in four.
The confirmation follows comments by Microsoft Game Studios general manager Phil Spencer, who said the game is "pushing" 3 million sold.
The original Gears of War has sold around 5 million copies to date.
Gears of War 2, released on November 7, has also hovered around the top spot for Xbox Live activity over past weeks.