I've found number three has brought so much joy into our home, the older two girls are 21 mths apart, then our son is 5 yrs younger than Dd2. They absolutely love him, he is so chilled and like Kaybaby said, it's so easy the third time around coz I feel very confident in my parenting and he just has to fit into the crazy family life. He developed his own routine pretty early, based around school drop offs etc, I think because there already IS some sort of routine in the home, they just kind of slot into it which makes for an 'easy' baby.
Also I just didn't stress about things like sleep, he catnapped til he was 6 mths old (for day sleeps) and with the first two that would've bothered me, but by the third I didn't really care coz by then I realised all babies are different and he'll sleep one day!
Congratulations and good luck!