I think it's fine for you as a young woman embarking on life to have had an idea of what family looked like to you, and for that to be 4 or 5 kids.
But to be so slavishly stuck on that regardless of your partner's wishes and how your shared life has actually played out isn't reasonable. It's understandable, but it isn't reasonable.
You AND your partner can change your mind on anything, at any time, and it needs to be OK. He didn't really want any kids, he had 2. He is fully entitled, for whatever reason, to think, no, I don't want a third.
If you do, and it's more important to you than your husband, then it may well be that your marriage is over - but it would be you instigating that. Because he is happy with you and his 2 children, and you are not.
I do feel sad for you, because it's not a feeling you are entirely in control of. The drive to reproduce is biological and not rational. But that doesn't mean you can't get on top of it and move on, if you want to. Because if you don't, you'll spend your whole life resenting him and feeling unfulfilled, and that isn't fair on either of you.