I'm definitely in the minority here but here is my opinion/take on your situation.

Your husband gave you an ultimatum to have an abortion or else he would leave. You decided against an aborting (totally your choice) and said you are fine with him leaving. Despite this, it's possible that he felt compelled to stay because of the two children you already have and ultimately decided to not leave for their sakes.

Your husband was given the option of having a vasectomy after the second child and refused (his body, his choice). But what other commenters seem to have missed is the fact that OP didn't specify (in either he post or comments, unless I somehow didn't see it) what kind(s) of birth control they were using, therefore it doesn't necessarily mean that he didn't use condoms, like others are saying.

There is also some information missing. What birth control methods were you using before getting pregnant? Did you decide how many children, if at all, you wanted before you got married? If so, was having a third child exceeding the amount you previously agreed upon? If so, he could feel betrayed in a sense, especially since he told you he didn't want a third child and I fear that it has lead to him subsequently resenting you which is evident from some of the information in your post, such as him refusing to be at the birth or help you out during pregnancy.

That being said, I'm in no way defending your husband's behaviour and lack of involvement with your two children and your pregnancy (after all, it takes two to tango which he seemingly doesn't want to admit or is ignoring). I agree with others that your relationship with your husband is not good and you should consider take up your parent's offer to live with them.