http://au.pc.ign.com/articles/918/918895p1.html
StarCraft 2's singleplayer component has been split into 3 games in order to make the game's storyline more epic. Each game has the same multiplayer component, but a different and distinct story mode.
They will
not be released all at the same time, but periodically and episodically. You could treat them as expansions, but they're standalone so you won't need the previous one to install the latest one.
Beta keys were handed out as part of the BlizzCon goodie bag, so expect the Beta to come in the near future.