I have to disagree with the 'you only regret the kids you don't have' saying - one of the reasons i don't want a third is the fear i would absolutely regret it. I'm sure i would love the child and give it the care it needs but it can imagine regretting it if it ends up being more than i can cope with (which it likely would be). I don't want to feel so exhausted and overwhelmed with my children that i struggle to enjoy them, i don't want to regret having that last child that pushed me over from coping to being in over my head.
I did hope to have 3-4 kids but i have to be realistic and listen to my head, not just my heart. I think i might look back and wish i did have a third but i also think if i did have a third i might be glad in 20 years time but what about all the years before then? The pregnancy, newborn years, 3 sets of school fees that need paying, the bigger house we would need to buy and all teh stress of the added costs etc etc - is it worth it? In my case i think it probably isn't - i feel happy with 2. I still would kind of like a third but not enough to go through it all again and possibly wish i hadn't of...
For me i will wait until i feel certain i want a third before i even entertain teh idea - doubt means don't do it (for me).
Give yourself time to really think about it. Good luck.