I personally love having a 5 door for my little girl who's now 3, BUT...
First 6 months they are in a travel system, that is easy enough to have on front seat, with airbag turned off which you can do on golf, and if you get an ISOfix car seat (the Britax one I have has been ace!) they are easy enough to install to avoid messing around with seatbelts(which are hard to fit in 3 doors) and th edoors open wide enough and seats go forward enough to have easy access to rear...
So basically a belt strapped seat is really hard to install into a 3 door so go isofix.
I've had numerous cars in last three years... even a convertible, and you do manage, but it's down to convenience and how many times a day you're gonna be a) fitting the seat in/out b) putting the baby/child in/out...
Dom
p.s. don;t have a civic type-r with recaros as they don;t move forward enough to get even a baby seat through to the back!