What episode in season 3 do they find out Alison is alive Pretty Little Liars?

Alison DiLaurentis is revealed to be alive in the Season 3 finale of Pretty Little Liars, titled "A DAngerous Game".

This episode is the season 3 finale, episode 25. It's the episode where the Liars are all in the dollhouse, and they find out that Alison is alive and has been controlling everything from the dollhouse.