I have always wanted 3 and am getting ready to TTC #3 now. I have a DD and DS who are 17mths apart, and am hoping #3 won't be any further than 2 years apart from DS. But I have had thoughts about #4 later on which would mean bigger car. But I'll wait and see what happens after #3.
You never know, you might find that after #2 comes along you aren't quite finished and do want more.
I can imagine missing the baby stage later on down the track and going for the 4th.