BingBong, I'm sorry that you feel sad. You have the disadvantage of intense maternal feelings, hormonal surges and just urges towards expanding your family. That's normal and you should accept it as such, you're not wrong to feel the way that you do.
What would you be advising a good friend if she were in this situation? If you could look at this as an 'outsider', you would probably come to the same conclusion that many posters have, ie. your husband has said 'no' to a third child, he has that right, he doesn't want to be stretched financially. You feel that a third child would be 'worth it'. The person who doesn't want the extra child will always hold the power of veto and they should. A child should be wanted by both parents and it wouldn't be, or there's a very big risk that it wouldn't be. Your husband would be very resentful of you and maybe of the child too. That wouldn't be fair and would place a terrible burden on your whole family.
You're staying in your marriage and presumably, are masking your longing for another child in front of your existing children. That's really important. They must not get any inkling of your desire for another child because they won't process that as adults but with the uncomplicated and black/white view of children.
There's also a risk of resentful feelings towards your husband, becoming more intense and possibly vengeful, that you are owed something you can't have and he's the one preventing the thing you want. It's not enough to stay in your marriage, it's got to be restored to harmony otherwise it's all for nothing. How are you going to do that? Do you think that it would be worth exploring your feelings with a counsellor and then with your husband? I think you really do need to be heard and by somebody who isn't listening with one ear and a finger plugged in the other one for fear of you trying to talk them around.
It's a fait accomplis, your family is complete and it's a perfect one. Now it's time for you to come to terms with that and get to the position where you are able to cherish what you have without longing for more. What do you want to do for you? Do you have aspirations of your own? Education? Developing new hobbies? Career development? Moving in to a new career and prepping the ground for that once your youngest child is in school?
Something for you now, BingBong, the time is right for that and I think your husband will be right behind you. Concentrate on what will make you happy and fulfilled.