Re: 3 accross
Evenflo SecureKid 300 / 400 for your 5yo should be perfect. Keep the younger 2 RF in the CA seats. That should puzzle well. I haven't tried the combo personally, but the shape of the shells should create an extra few inches for you.
Do you have a babies r us or BuyBuy baby near you that you can try the seats in your car before buying? No obligation to buy from those stores, but they will allow you to try any seats in your car.