Have you thought of just doing three of the narrower/narrowest car seats and/or booster seats, or combo? The center one would just need to 'share' the two inside latch anchors of the car seats on either side. Or one or both of the outside seats could use the alternate seat-belt attaching method as almost all car seats are duel - can use latch anchors or the seat-belt.
The narrowest car seats are about 17 inches, and the width of the back seat, at its widest is just over 51 inches so a 'theoretical' fit. The armrests and curve of the door toward the seat take away from that, but depending on the shape of the car seats, and where the armrest hits them, could work. Might take some testing of a few at an agreeable store to see if three would actually fit. but might just be do-able.
2015 i3 Rex, Capparis White, Tera World, Technology & Driving Assistant, Parking Assistant, Harman Kardon Audio System, 19 inch 427 wheels, EVoInnovate EVSE