N.M.
My heart goes out to you, I completely understand the conflict that you are feeling. Part of me would love to have a 4th child, but we are so financially strapped I would be devastated and happy all at once if I were pregnant. You and your husband obviously love one another, your family and you unborn child. Right in that you have all you really need. It seems to me that you were destined to have this baby. I don't know if you are spiritual or not, but there is a saying that God gives you as much as he knows you can handle. He has faith in you. Please have faith in yourself and one another.
I have a few suggestions that might help. First, by the time you have your 3rd, your oldest should be ready for preschool. Some school systems have free preschool, look into that. If not, see if your city has a head start, that is often sliding scale. Daycare is often cheaper the older the child is, this way your mom could still watch the other two. Don't be embarrassed to apply for assistance such as WIC which covers formula, milk, bread, cereal, food stamps, energy assistance, child care assistance and even health care assistance. Church is often the best place for support, emotional and otherwise. I clothe my kids with hand me downs, Salvation Army and Goodwill, and none of their friends can even tell. I do the same for myself and my husband. We shop with coupons, in bulk and at stores like Aldi and Save A Lot. You can do this!!