+1 to Lhamo's idea of seeing if the grandparents will help fund college. When my daughter was young, my father kept mentioning he was going to fund a 529 account for her. At that time my father was >70. I finally said, "Dad, it is really generous of you to fund X's college, but until there is actually money in the account, we can't plan on that." My dad, bless his heart, funded the account that year. Now every semester I remind DD how lucky she is to not have to take out loans because of her (now deceased) granddad. I would not have felt comfortable unless my dad had mentioned college funding several times himself, but I think it would be reasonable to tell your wife that would ease your concerns....if it in fact would, and see if she is comfortable talking with her parents.
On the other hand, it is hard for me to imagine making a decision about having a child turning solely on money. Are there other issues for you? More baby care? Having kids spread over a wider age group so they are in different schools, activities, etc? Or you just don't want three kids? Those all seem perfectly valid to me and I agree with all the posters who suggest that if both parents don't want another child, they shouldn't have it.