I've got two second stage seats and a booster seat (with a back) in the back of ours at the moment, so don't see any problems with your three. Mine are all britax btw, some brands had buckle crunch problems.
Ooh, it's fab. The best thing is the sliding doors, makes getting everyone in and out soooo much easier. High door frames too, no hitting lo's head on the way out. and there's so much space to put everything in!
Not the nippiest of cars, obviously.