My situation is a bit different. My ds's issues are more medical in nature. I can relate to charlottes thinking about appointments and medical expenses.
For us, and its incredibly morbid, but our thinking about a third was the knowledge that we have no idea how long ds will be with us. He could live another 60 years or we could lose him tomorrow. I couldn't bare the thought of dd being alone once dh and I were gone. We always wanted three but she was really what swayed the decision for us. (That and an oops pregnancy. Lol) so for us I guess, ds's special needs made it more important that we had a third.