I cant imagine lying in bed at night and regretting #3 child (if it happens). I would hope no one would ever 'regret' their children. I think its more a case of not liking the situation your in, rather than the child itself.
Why dont you get a dog, or plan a big holiday for the 4 of you? SOrry, i dont mean to sound harsh, but i kinda agree with Fionajill. The whole fact thats its a hard decsion for you should maybe see you and DH compiling a list of positives and negatives??

I used to ask parents with children close together 'ohhh, how do you do it?'. I remember one lady replied 'you just do, you dont have a choice really'. That said, now having my two close together, i realise you do cope. YOu cant sit in a corner and scream i cant do this anymore. As adults we need to be there for our children all the time. Thats how i see it anyway. I would prob ask yourselves what you feel another child would bring to your family? For some families, 2 children work great, and they are perfectly happy with there 2. Other families want 3,4,5....for whatever reason.
Good luck hon, with whatever you decide....