I'm in agreement with rockstache. I was the middle of three, and honestly, I would never have 3 kids. 1,2,4,5, whatever, but it was truly awful being the middle of three. (When you're the middle you're the greatest competition for both the youngest and the oldest, so you end up being very disliked). I took the hatred of my sisters and became a very accomplished and strong individual, so I can't say it was all bad, but I would never have 3 children because of what I went through.
However, I think you were talking about financial concerns, and for that everyone gave you good points: college costs, child care costs, vacation costs, etc. Not sure if anyone brought up the possibility of needing to upgrade your car to fit 3 car seats, but that's something to consider as well.