Although I have zero kids of my own and am plenty happy about it, I have many friends who have started families, a few with a small herd following them around. They all attested that going from two kids to three was the hardest — going from man-to-man to zone defense is a big switch apparently.
As much as that change might be difficult, that doesn't necessarily mean that they're unhappy. In fact, according to science, having more kids makes you happier than having fewer.