I have 3. Two boys (almost 6 and 4) and a girl (2). I love my family - wouldn't change it for anything. My boys are inseparable, play great together, enjoy each other, and share just about everything. They are creative in their play and entertain each other inside and outside. My daughter is a spitfire and she keeps me on my toes. I'm 38 and we thought about a 4th, but for a few very good reasons are sticking with 3. I "got my girl" and I am happy. My oldest and his sister are great together. He is very patient and shares with her, helps her, and loves her (tells me all the time how cute she is).
The other day we were at the YMCA and my kids were all in the kid area/gym and they have a climbing area for older kids. The daycare ladies let them all go in (even though my daughter is too little) and the boys would boost her her up so she could go down the slide. It was adorable and makes me smile everytime I think about it.
That said - some issues we have: my second son (age 4) wears me out many days. He plays well with his sister about once a day for about 30 minutes. The rest of the time, they taunt and tease each other, and bicker WAY more than the boys ever did when they were 4 and 2. Maybe it's a boy girl thing, maybe it's birth order, maybe personality... but it's a challenge for me.
When I am out with all 3 kids (especially if we are out after lunch) it is often a challenge to keep them together, moving in the right direction. When I have two, it works much better.
I'm out numbered when getting kids in coats, shoes and out the door. My most difficult times of day are when we are coming or going with all 3 kids. I assume this will get easier as they get older. Having 3 kids in 4 years was amazing and exhausting :)
Tonight my boys were playing great together, and every time my daughter was in their space, there was a problem. I removed her and then they were fine. I felt bad for her for having to be away from them, but it worked better. She's starting to skip her nap and is just way to tired to function at the end of the day. That is a challenge too.
We were in line to move to a bigger house either way, so adding a 3rd only helped us move that along faster (it worked really well for us timing wise - we lucked out). We also had plans to upgrade to a newer/bigger car right about the time I got pregnant with #3 so that wasn't an issue either - as we had budgeted for moving up to the minivan. We do only have one car though - so sometimes that is a challenge.
We've started saving for college and that is a little daunting when I think about how much is it going to cost. Currently we are saving $300 per month toward their education. Hopefully we'll be able to budget more in the future.
My daughter doesn't get all the "mommy time" the boys got, she wants to play with the big kids and doesn't really seem to have time for me at this point :) This is good and bad... good when they are all getting along.
Good luck with your decision!
J.