if there is a hook there already then i assume it must be safe and legal to have a carseat in the folding seat.

I have two diff car seats and they didn't fit with DDs on side but did with hers in the middle, so it might be a bit of trial and error to see how your seats fit.

i found it easier to have bub on the side, when doing them up. and dd could climb around him or get in other side.

i don't know if i could even fit 3 seats in my car, would probably have to find 3 slim ones if we were in that position.