I think you really need to sit down and think about what is best for your existing family right now.
I agree with the previous poster. Is having a third child the best choice for your two existing children? I know there will always be posters who'll say "every child is a blessing, your existing children will love their new sibling" but that's simply not true, not in reality. And if having a third child leads to the breakdown of your marriage, or even leads to a rocky few years, I couldn't put them through that either. Yes, I'd feel grief, and guilt, and resent my husband for putting me through that. But it's not about me. It's never about me, it's about my kids.
If having a termination leads to such resentment, such guilt and grief that it breaks down your marriage then so be it. In my view you will have done everything to spare your current children the pain of a divorce, but the divorce was unavoidable.
Other posters might disagree but I think there's a much higher chance of your marriage breaking down if you go through with the pregnancy. And it will be much tougher on you to be a single mum to three kids under four rather than two, even if you are doing it free of the guilt and grief of an unwanted termination. Believe me, the breakdown of a relationship leads to a lot of guilt and grief anyway.
I think if you decide that continuing with the pregnancy is the only route you can take, you should take it with your eyes wide open. I don't know what age you are, but would you be able to cope if your third child had additional needs? I think you need to understand that your husband will place the blame for the breakdown of your family firmly at your front door, and there's a good chance he will act accordingly. Will he be totally frank about his finances, especially since he is self employed? Can he take the kids 50/50 if he is self employed? Can you afford separate living spaces? Would you have to move in with your parents? How would you explain it to your kids when they are older?
I come to this with some experience. I have three kids, the third being a "surprise" after contraception failing. I wanted a termination initially, my husband didn't. I decided to go ahead with the pregnancy because we were financially and emotionally stable. Now don't get me wrong, I love my third child wholeheartedly. However, he is autistic and the first few years were incredibly tough. On balance, it was the wrong decision for my older two children. There's no getting away from it. You don't have any free time after you have two children, so in order to make time for the third you steal from the time you were giving to your first two. That's generally fine, on balance they have a third sibling to play with and rely on when they're older, so that makes up for it. But the amount of time, and finances, I have had to give to my third has led to such resentments from my older two and it's only now, ten years later, that things are evening out. And they don't get a third sibling to play with or rely on when they're older. They will have to take care of him when we are gone. I can't imagine rolling in a marriage breakdown in to that, with the emotional, financial and time tolls that that takes.
I know this is probably all you are thinking about, but don't let your heart rule your head right now, because trying to save yourself a broken heart right now might actually lead to a worse broken heart in the future.
Maybe going to a counsellor might help - even an online one?