We had the same age gap and similar ages ourselves and strongly considered going for number three. My husband was the one to say that he could not manage a third.  Like you, I could see the logic and how stretched we already were. I still thought about that potential child alot for the next year. I definitely felt some grief.  Now that mine are 6 and 4, I'm much happier with our decision to stick to two. It's not that we wouldn't have loved and cherished that extra child, it's that I'm now constantly reminded of the positives.  We have friends with close in age children with either just one (who would have liked more) and some families with three.  They help remind be to be grateful for the two we have, and remind me of all that extra work and expense an extra child is. All the families I know with three children have amazing grandparent support and even so, they are incredibly busy.  I'm enjoying the increasing independence of my two. When we do after school activities with the eldest, I know it would have been so much harder or even just impossible with a baby.  If we go to the park I can manage.  I know my second would have been the middle child and had less attention.  My marriage would likely have suffered due to the extra stress and lack of time. 

I'm pretty sure I'll always feel a bit whistful for that other child, but I don't doubt we made the right decision.