All 3 were "accidents." #1 was conceived within 2 weeks of missing a dose of Depo Provera, and #2 & 3 were both conceived during vacation when I forgot to keep track of my cycle. I think I just take after my mom and am very very fertile. Dh's parents had fertility testing done and found FIL had a high sperm count, so I wouldn't be surprised if dh did, too. (MIL's inability to conceive was due to stress they found out, cuz there was nothing found to be wrong and she conceived within a month of quitting her job. LOL!) Like my FIL likes to say, "You can convert the Catholic girls, but it doesn't diminish their fertility." LOL!
edited to add that most people I've talked to who had trouble conceiving their first had no trouble w/ subsequent kids. Although there are exceptions, this seems to be the rule, at least IME.