Valve’s Gabe Newell spoke at Games for a Change today in New York City and there he said that Portal 2 has reached over 3 million copies sold since it was released on April 18th. He didn’t actually mention what platforms had sold the best or whether or not this was sales on one specific platform but it can probably be inferred that he meant all platforms.
Gabe Newell is most likely the best source of sales data because Steam is usually quiet about sales through their service and Gabe actually has access to this data. When Portal 1 was released it had sold four million total copies between its release and Portal 2’s release. It’s always good to see a game sell well, which means that there could be another Portal title in the future, or maybe finally Half Life 3?