I would go into the baby shop too!!

Personally I upgraded my car, but that was always my plan, I did not want to be fitting 3 carseats into a Daewoo Nubira wagon!! They are too narrow!! and I struggled to get into the middle past the 2 older children so there would be no way I could get past with a baby.

I would recommend using a capsule so that you can put the bigger boys in with as little disturbance to bubs as possible. I would also have bubs on one side so that you can easily get all of them in and out. I also find the capsule a bit narrower than the convertible carseats.