We have two, and since our experience was kind of "different", can't say what we'd do. My first son died and the ob-gyn said I'd never have full-term babies. Well, I switched doctors and had 2 full-term healthy boys. I'd always wanted 3 children, but decided to thank God for having my two healthy sons and not chance any more.
However, one of my sons and his wife had the same dilemma as you, I think. They had 2 girls when she was in her mid-20s, and he was almost 30 when their second was born. She really wanted a third, but he was happy with 2. When she reached 35 and he was 40, they decided "now or never", discussed it (well, did more than discuss), and about 1 1/2 years later, she got her third. So their kids are 13,12, and 2 now. He's a 43 yo with a 2 yo, but she's really a joy.