This is really hard, OP.
Something similar happened to us, and because you and your DH feel differently about the pregnancy, only one of you can have what you want. Considering its your body, it is, in a way, your choice, but that will be hard for your DH who, effectively, doesn't get a choice.
I know that when it was happening to us, I did have an underlying and never expressed feeling of annoyance that he was responsible for his own fertility, and should live with the consequences of his actions.
Financially, having a third DC has not been an easy choice for us. Like you, we have had to extend our house. Holidays are different, because taking three needs different room configurations in hotels etc. (So we usually self-cater.) We have had to choose different cars than we would have with two. Obviously, three will be more expensive than two, as far as schooling is concerned, not to mention the cost of extra curricular activities, clothing, socialising, etc.
It really wasn't easy in the early years, because I always had the feeling that DH resented DC3, and was very sensitive to that. Even if it wasn't true, if you worry that it it is true, it will still cause problems, IYSWIM.
DC3 is now 10yo, and while I acknowledge that life would be easier without him (as it would without any child, if we're honest) he is such a joy, and I am so pleased we did it my way.
I don't know if this is helpful, but I really do feel for you.
((((hugs))))