Prius - how much space on back seat?

The short answer to your question is, "probably not" -- but child seats differ greatly in width, and it may be possible to find 3 that do fit.

The back seat is OK for 2 adults and a child in a booster, but just barely. I just measured ours (2004) and the clearance from armrest to armrest is 50-3/4 inches, with the possibility of a small measurement error. But recall that you need some spaces so as to access the "sockets" that the metal tabs plug into. Edmunds lists the width as "Rear Hip Room: 51.6 in." but I measured at the widest part of the armrest, not the part closest to the seatback, since most child seats are rigid and not tapered fore-to-aft.

If just a booster is needed in 1 or 2 positions, styrofoam blocks with a light cushion wrapped in a pillowcase would probably work as well (the goal is a bit of height so the lap part of the belt goes across the thighs and not the tummy, and a shorter seat so the legs can dangle w/o the child scooting forward from the seatback). Such homemade boosters could be made skinny. That'd be fine for modest trips, but crowded for cross-country tours.

Hope that helps.