in all of the Disney buses, there is no way to use a car seat. a lot of taxis do have a car seat, and you can always have one requested when calling for a taxi to guarantee there being one. although I didn't want to deal with brining a car seat, at 3 I would think about bringing it just for the airplane alone. I know it was much easier keeping DD in her car seat on an airplane vs in just the lap belt. she also was more comfortable, as the car seat is familiar to her.
This year, we are NOT bringing one, even with a small trip planned off property via a friend's car. At 5 years old, a car seat is no longer required, just recommended. (she still uses them at home, just not going to lug it around for one short trip.)
So, do you *need* to bring one? No. should you? well, the only times you could use it is for the airplane, and in a taxi/rental car. since most taxi's have ones to use, I don't think you need to drag yours along unless you want it in the airplane.
but, as always, the choice is a personal one. (that long magical express trip on the bus, has no seat belts and you will have to stow the seat under the bus with any stroller you may have)