Hi Catherine - we had this dilemna too. We had decided that 2 children was enough, and then one day we changed our minds. Life is short and our family is everything to us, so we thought we could definitely love another child. It really is a decision you both have to agree on though. Our original decision was based on what we wanted to provide for our girls - Good schools, lots of extra-curricular activities etc. We then realised that money isn't everything and we can give our children a happy life with lots of wonderful memories without having to spend heaps of money.
We too have 3 bedrooms, so our older girls will be sharing a room, and bubs will have her own room. (until the eldest turns the age when she wants her privacy lol)
Good luck with your decision.