How old are your DCs? Sorry if it's already been mentioned, I couldn't see their ages anywhere though.
We have two and had always talked about having three but we had difficulty conceiving initially then an early mc with our first, followed by DS, followed by a late mc, followed by DD. They're now 3yo and 18mo.
When DD turned one last year I mooted having that number three we talked about, not necessarily there and then but just a discussion about it and to my utter dismay DH said no. He didn't want to try for number three, didn't want a number three and was instead happy to devote himself to the two we do have.
At first he refused to discuss it until one night after we put the DCs to bed I asked him to explain his decision to me. There were tears, more than a few recriminations, I said he was being unfair,he said I was being unfair, and for the first time ever we had a serious talk about splitting up. It was only a thirty second talk, as soon as the question was asked we each said we didn't want to split up - he didn't want our marriage to end over it and I don't/didn't want just a child I wanted his child so going off and having a child with someone else would defeat the object.
For two hours we went back and forth over it and in the end we decided to leave it alone for three months and discuss it again after Christmas when we'd both had time to think it over and consider the others viewpoint. I knew inside that if he came back and still said no I'd have to live with it and that actually I could be happy. I love my children very much and I'm thankful for them so I could take comfort in that at least.
A few weeks later, one night in bed, DH whispered to me that he did want a third but he was scared. He's seen me at my most broken, crying over babies we couldn't seem to manage to conceive and then crying over babies we lost. He's been broken too by those same things and he told me he's felt guilty about it for years because in his mind he did that to me, he put me through that heartache. He was sorry for the pain I had delivering DS and the EMCS I had with DD. He had all sorts of irrational worries about me dying in childbirth or us miscarrying again or having unexplained infertility again. We had ourselves a talk of a very different kind and I realised he'd been keeping these worries to himself for years.
He also told me that DS and DD are so very young and this was another reason behind his decision not to have another. We're now at a place where we're going to start TTC in the summer once I'm settled at my new job, by the time we conceive and deliver DS will be in full time school and DD will be at nursery half days, an altogether more manageable prospect than three under fives at home all day long.
Talk to him, find out what he's feeling. Don't go in there all guns blazing, livid about his decision - YABU to react that way - but do tell him how it's making you feel. Ultimately you can't have half a child, this isn't an issue you can compromise on so it's all or nothing. You may need to ask yourself which you want more, him or a third child? If the answer is him then you need to abide by his decision and make your peace with it. If the answer is a third child then you need to end your marriage.