Since Atatsuki mentioned that there were 2 other Jinchuuriki that weren't missed when they were taken from their villages, counting Garaa and Naruto, there are at least four known Jinchuuriki in the series so far. (Alive or dead.)

Then there are the other 5 Biju, I have no idea where these are, but I know that each Atatsuki (other than possibly the leader) was assigned to catch one.

Wait.. but I remember the old lady saying there are only three Jinchuuriki.. I guess the when that Atatsuki member said two Jinchuuriki weren't missed, he was talking about Garaa -and- another dude. I guess we won't see the other Jinchuuriki then. Since he's dead.