You never regret the children you have, just the ones you don't!!
Sometimes you have to go with how you really feel, money is important of course, but how do you REALLY feel?? You will find ways to stretch money, and of course once they are all at school you are able to work again (if you don't now). Will you regret not having #3 in 5-10 yrs time?? (hard to answer I know)
We are 17.5wks with number 4 (we had our twins first

. We will have 4 kids under 3yrs 7mths, but we just KNEW it was the right decision for us! Our family didn't feel complete. We will struggle sometimes with money, but I am working on our budget, and how we live to cut costs as much as possible! Cloth nappies, cooking from scratch, preserving excess fruit etc for winter, making my own cleaning products....all help keep the cost down.
Good luck with your decision, it is a hard one!