I find this quite a weird question! Not that there's anything wrong with having three (or more) other than the environmental considerations, but I can't honestly see what family advantages three would have over two. I mean, I understand every child brings joy, but in a mathematical calculation, I can't see what difference it makes.
Two seems like a perfect balance to me - sibling companionship, no only/middle child syndrome, more money to go around, don't have to upsize vehicle/house, if you work it's less effect on your career. For a third, I'd be worried about getting twins and ending up with four.
Security-wise, two is also so much easier to manage. I know this probably sounds dramatic, but if I was out with my two kids (3 and 5) and I found myself in a dangerous/emergency situation, I could pick one child up in each arm and get out of there. A third kid and I'd be struggling to hang onto them all.
Same could apply with trips to the swimming pool, beach, amusement park, into the city etc, where you have a lot of hazards and need to be able to keep smaller children basically within arm's reach at all times. Unless you have another adult with you, the logistics for that sort of thing becomes really challenging with three, so it restricts your world.
I only ever wanted two, and even though I had easy pregnancies, never felt tempted by a third for one second.