This sort of thing makes me bloody angry because clearly having another child - or not - is a pretty black or white situation, one where it's impossible to compromise.
I'd have more respect for him - and I think it'd be easier for you to come to terms with - if he'd sat you down and regretfully explained that he'd changed his mind since originally 'agreeing' to try for two children with you, and then, actually took the time to explain why that was .... e.g. money concerns, accommodation concerns, him finding 3 children a struggle or whatever.
Instead, he's readily admitted that he lied to you .... that he pulled the wool over your eyes regarding his intentions, and then unilaterally changed the goalposts so HE got what HE wanted.
I know you say that otherwise you're happy but I do wonder how you - or anyone - comes to terms with being tricked, which is a lot different IMO, to someone changing their stance on an important issue with genuine reasons to back that up. My fear for you would be that this is the sort of thing you don't easily forget and how much it's going to niggle away at you in years to come .... which of course might well lead to resentment and also affects the trust you have (or not) in him as you may justifiably start to wonder what else he may have lied to you about and how genuine he's being about all sorts of other things.
And of course, the step issue is an added complication which makes something like this even harder to cope with. It's pretty damn insulting to suggest that you should be 'happy' with the three you (plural) apparently have. I know you know why that is, but not least I should imagine your stepchildren's mother wouldn't be happy if you started to act like their 'mother' - assuming you even want to. Sorry, but I think his attitude stinks and is very arrogant ... that his children are so wonderful you should be more than content and abandon all your hopes of another child of your own so you can bask in their presence. Doesn't work like that.
As others have said however what this boils down to is either accepting the situation as it is - or leaving him to give yourself the opportunity (maybe) of eventually meeting someone else and having another child with them. It's not a great choice and I really don't know how to advise you .... it's one thing 'coming to terms' with a situation you don't like which has resulted from sheer bad luck or a combination of circumstances which can't be helped, but quite another to try and do the same where your partner has deliberately misled you over something you'd made crystal clear was really important to you. I'd personally see it as not necessarily coming to terms with having no more children, but coming to terms with the fact he'd lied to me and tricked me. Had he said right at the start he wanted no more chances are you'd have walked - allowing a serious relationship to develop and then have one child quite obviously entangles you in all sorts of ways (emotionally, financially, practically) that make it so much harder to move on if you wanted to.
I'm just very sorry you're having to deal with this and perhaps the best suggestion I can make is some personal counselling for yourself so you can explore your feelings, and your capacity for forgiveness etc., before you make a decision one way or another. Sorry I can't immediately recall your other threads but if, as a PP has suggested, your own child is being pushed aside in favour of his older children, then that makes his stance even more insulting and insensitive, and for me, personally, would be another reason why I'd be thinking very hard about whether or not I could truly hack a future with him.
Am sorry if I've been too blunt - haven't intended to upset you. I just feel very angry for you.