You don't need a car seat for any child on an aircraft. You can choose to take one but they have to fit very specific criteria. They have to be forward facing, able to be correctly fitted using just a lap belt and actually fit in the aircraft seat.
If you need a car seat at the other end, it's usually easier just to put it in the hold, or hire/borrow one when you get there.
A 3 yr old will have their own seat and just be restrained with the lapbelt, the same as an adult.