L.M.
Without personally looking up each carseat, assuming the each child is within both the height and weight guidelines for the carseats they will be using I think this is what you should do:
1. Since the youngest child is still rear facing, he should go in the middle of the back seat and remain rear facing if his size permits. This is the safest place for kids and since he is youngest, his back/neck is the least developmentally strong and should be there.
2. 3 year old should be behind the driver seat and be forward facing as his size dictates. Behind the driver is statistically safer than behind the passenger seat and should be here if middle seat is taken.
I would guess that the ones with side protection are safer than those without.