Everyone already knows that Half Life 3 was confirmed by the new Techies hero in Dota 2 [http://hydra-media.cursecdn.com/dota2.gamepedia.com/c/cb/Tech_rare_05.mp3]. It's no coincidence that there's three of them.
Really though, I doubt it's legit. Half Life 3 is little more than a joke by now - even to Valve, who, as mentioned, makes a reference to the "Valve can't count to three" gag with their new Dota 2 hero. ASUS probably made the joke to say that the card wouldn't need upgrading for a while.