How in the world was he planning a triad from the beginning by lying and cheating? Poly is about living honestly and having multiple connections the ethical way, not cheating.
Didn't you say you were dating for months before you found out he was even in another relationship? How did that make YOU feel? What made you decide that an omission that big was okay? He lied to you about having a fiance (not mentioning it might as well be lying in my book). He started rumors about you that made his fiance dislike you. He broke up with you. What are you seeing in this guy? Even if he is the nicest man on the planet 90% of the time, the other 10% he is capable of these hurtful, relationship-ruining things. Do you really want to spend your life with someone who could decide to sabotage the whole thing tomorrow?
I know you've said that his fiance doesn't know that you two had a romantic relationship (and that he specifically lied about it to her). Read around here, one of the things you will see over and over and over again is that poly doesn't work without honesty. Until that can be established, there is no hope.