Advice from those with 3 kiddo's under 4?

[Trillian]

My first three are all 20mths apart and I loved it. You find your 'groove' really easy with the addition of the third bub as like purplemamma said you're already doing all the baby stuff still. I found though that come the third bub, I was such an old hand by then LOL that she just fitted in perfectly with the routines the older two kids already had, so there weren't any dramas with feeding and naps being out of sync with one another too. Plus you can often convince the eldest to still have an afternoon nap if you need a nap yourself so all of you can have a nap together.

it really is great and I reckon you should go for it, but that said, the only down side is they all go off to school very quickly once they are that age LOL - ours have a two year school gap (except for Alister because of the bigger gap), but it still goes very quick

[Cricket]

I think every situation is going to be a little different, due to the different personalities, needs, abilities, etc, of each child. I have a four year old, two year old, and baby, and it's just fine. Of course, I haven't tried anything different, so it's not like I can compare, but for us, this is not a problem at all.
I think there are pros and cons to however your children end up spaced, and there will be days when everything goes beautifully and days when you want to pull your hair out regardless of how old your babies are. If you want another baby, go ahead and start TTC - I don't think you'll hear from any mothers on here who regret having another one, regardless of how close or far apart they are. As soon as that baby arrives, you'll know they belong in your family, and couldn't imagine it otherwise.
All the best!