Three's company.

I can remember on more than one occasion hearing parents describe the addition of a third member to their family as being the transition that "broke" them. They related, "Once you have your third child, you're already outnumbered. You might as well have a dozen kids at that point. It can't get much harder."
Those people still had three kids at the time. Oh, and they were wrong. Sure, three kids is most definitely harder than two. But it was manageable. As the mother of four children, I can officially testify that four kids is infinitely harder than three. We've lost all control around here, as evidenced by several pictures I've managed to snap in between pulling out my hair and barking orders at my kids. I'll post them someday. But since grabbing a minute to myself is a commodity in short supply, I can't say when.