David McCusker writes saying that he has heard that three children are easier than two and speculates that it is because the oldest child is able to help with the younger. John O'Sullivan wrote back to assure him that three children is not easier than two. John also links to the book The Best a Man can Get, about a man who escapes from fatherhood while his wife copes, saying that any thirty-somethings considering children should read it.
John Sullivan is probably right that for those men over thirty (or perhaps even 25), having children is something that is extremely difficult to adjust to. You've got freedom, you've got mobility, you've got very little responsibility. Having one child changes your perspective no matter what your age. Having a second one soon afterward (in my experience) only serves to reinforce the imposition.
If, by the time the third one comes around, you still haven't adjusted your expectations, then the third one can be just as bad. This is probably what John means when he warns off "thirty-somethings" from fatherhood.
I think, though, that if you start having children when you are younger (I was 23), you can adjust your expectations easier than if you start having children around, say, 32. This is probably why David has heard that three are easier than two — I really haven't notice my five-year-old helping with the three-year-old and 20-month-old. She's just bossy. She could probably be more helpful, but that will come with age.
People who recognise early on that children will impede their goals take steps to avoid them. Richard Stallman told me this is why he hasn't had children — it would get in the way of his drive for Free Software. Dan Lyke has said that he has avoided children because he was told repeatedly that they are an enormous responsibility (I'm paraphrasing).