Valve's Game Newell talked to Games For Change in NYC, announcing that their recent release, Portal 2, has sold 3 million copies since its launch during April 2011. No platforms were mentioned, although it seems more than likely he was referring to sales across all major platforms - that means PC, Mac, Xbox 360 and PS3.

As Joystiq notes, the original Portal game sold 4 million copies since Portal 2 hit the market, even though those figures don't take Steam into account.

The fact that Portal 2 met with successful sales makes us happy because the game deserves it. Personally, I would've preferred a slightly lengthier single-player campaign, but who the hell am I to complain so much... So, fuck it. Portal 2 has enough juice in it to keep any gamer going and it also has a kick-ass co-op mode, which makes it a great purchase.

Source.