Valve's Gabe Newell casually mentioned sales of Portal 2 during a talk at Games for Change in New York City. Three million is the number he told the audience, as captured by Joystiq.
While Newell didn't offer any detail about retail and Steam sales, that is an incredible accomplishment given the game's April release and full retail price point. By the time of Portal 2's launch, the original game had sold four million retail copies. At this rate, it seems likely that the sequel will match the sales of the original. And with the money made from Portal 2, Valve will finally be able to complete development of Half-Life 2: Episode 3. That's how it works, right?