I think that there's a lot more to your question than just working full time with three.
Have you talked honestly and openly with your current husband about your fears of a third pregnancy? Even the illogical fear that he will turn into your ex-husband? If you can get past that piece (and by "get past" I mean come to a place where the pregnancy fear isn't holding you back, not that it's unreasonable at all), then...
People have three and work full time the same way they do it with 2. Yes, it would be different to work full time when the new one is a baby, but so much about this child's life would be different. You'd be busier, but there would also be the extra joy of another child. There isn't a right answer here, but I guess you just do it, love your kids, and know that you're doing the best you can for your family. That's all there really is, right? Good luck.