You can get by with a 3 door but a 5 is going to be easier.
We switched to a 5 door GTI (from a 2 seater) because we have a young baby with another on the way. Yet, I grew up with my folks having two door cars (Triumph Herald and a couple of Alfa Romeo Alfa Suds) and we got by ok.
One tip..... because we live in a small London flat we take our now 8 month old out of the baby seat and leave the seat in the back of the car. We just dont have the space for it in the flat. Out of town I see most folk take the baby seat out of the car with the baby in it when they get home. Baby might stay asleep if you are lucky, but you will wake them when you take them out of the seat so only delaying the inevitable and young babies shouldn't spend too long in a car seat, it's not good for their backs. Just take baby out of the seat and leave it in the car.... less weight to lift and easier to get in and out of a car, especially a 3 door.