 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. | 
