Only problem using a car seat in the st3 Is the seat belts are a bit short in the back and no option to turn the passenger air bag off if you want the seat in the front.

Have my niece in the back of mine a fair bit, luckily the seat base is narrow so fits in the rear seat perfect. But the belt is very short and need to tilt the baby seat a little to fit the belt correctly just for an extra inch.