I think you are going to need to buy 1 or more new seats to get everyone to fit well
What are the seats that you own now?? Also try latch and seat belts... (not at the same time) I mean if it doesn't fit well with latch try the seat belt instead... it makes a difference with some seats
I would put your son in the center and then rear face the other 2 on the sides
Radian 65 or 80 are very narrow seats
or infant seat center and something like 2 evenflo chase's outboard
Infant seat center, 2 marathon's on the sides
Graco Nautalis for your oldest, Radian 65 for your 1 year old and a snug ride for the baby
And as a side note... have you done any research on extended rear facing? The AAP states children should rear face to a minimim of 2 years now. It would be safest to flip your little girl back around.