Thank you all for your replies. It really does help to hear people's objective opinions.
I've definitely done some soul-searching on the matter along with some good healthy introspection. I truly do not feel I was intentionally being manipulative. However, I did have a lot of unanswered questions and suspicions that never got aired or resolved because most of my correspondence with this man was through text and Facebook Messenger. It was fully my responsibility to communicate more effectively with both of them, I just didn't know how or if my concerns or issues were even considered important to either one of them.
All in all, it was a huge communication failure. I'll own my share and learn from it. The truth is that despite my past relationship with this Man, none of us really knew each other very well at all. Personally, I don't feel that a few months of Facebook messaging is enough to plunge into a complicated, emotionally-charged poly relationship like the one we were attempting. The poor wife must have felt steamrolled.. having spent 18 years with this guy and all of a sudden, I show up, his "lost love from the past". I'd feel threatened and anxious, too. There were a bunch of mixed signals. At no point did all of us sit down and have a good, long talk. It was just a mess.
I'll take personal responsibility for my part of it, as well. It was not a good idea to attempt any of this while battling major depression. The next time I attempt poly, I'll be in a healthier, better place.
My one regret is having to walk away from this after things ended so badly. I wish we all could have just sat down face-to-face and hashed all this out but that's just not going to happen. Live and learn, I guess. Again, thank you all for your advice.