I can't believe how insensitive some of these responses are! How can someone think that a puppy could replace a child?????? I'm in a very similar situation. My son is 6 and in Kindergarten, and my daughter is 3 and starting preschool in the fall.
The best advice I've gotten regarding having a 3rd child came from a blog post I read a few months back. Ask yourself this: In 5 years, will I regret not at least trying? My answer was an instant YES!
My husband feels the same way, but we have some hurdles that we're having a hard time overcoming. We already have all the clothes for a boy and girl, we have all the toys, carseats, crib, everything like that. However, we'd need the money for our home birth (insurance is way behind on covering things like that, and I won't birth in a hospital unless absolutely medically necessary), and we'd need to seriously consider a bigger car (fitting 3 car seats across the back of our Escape is challenging when everyone's in the 5pt harness, let alone 2 in seat belts). Those are two HUGE expenses that we're having a hard time finding a way around.
Absolutely we would if given the opportunity, but the other thing holding us back a little is the fact that even if we get pregnant today, I'd be 36, and he'd be almost 39 when the baby is born, and that's not something we're sure we want. If I were 30, I'd probably be pregnant by now :D
Good luck with your decision, but I suggest you pose that question to your husband about how he'll feel in 5 years if you don't at least try. He may surprise himself and say that yes, he'd regret it. However, be prepared if his answer is a quick resounding no.