NRE and decision making.
triads are a mine field. poly-fi triads quadruple those mines, and create a breeding ground for upset and alienation for whoever is not in NRE. The "unicorn" is gunna win, hands down, as the nre is going to be between him/her and one of the couple - the left out couple, if unprepared for what's coming, is going to end up pretty dang unhappy. This seems to be the truth that i've seen, both in my experience, and in the experiences of other reading that i've done. a lot of relationships end because of attempting to have a poly fi triad, and the two people in nre often end up giving things a go. It's a surprisingly common & bizarre conclusion to an idealistic mission!!
Once you've been 'round the block, you know what to expect (often, peeps don't listen to advice, which is all over this board, lord knows i didn't!). When you're starting out, it's quite a wake up call, and an unstable relationship can go south in a hurry.
My hubbo and i were rock solid, and our poly-fi triad was ripping us apart - same thing happened, xgf and i started having distance despite our budding love, and it went sideways from there. lucky for us, my hubbo and i are both super committed to each other, because there were some mega complications in our sitch too. Whatever steps we take to move forward from here will be after time, therapy and self-examination. if we'd kept moving forward, it would have destroyed our marriage.
ideals vs. realities - there can be a big discrepancy. the reality of your sich is that the poly-fi triad ended in divorce - not quite all equal in the end as the ideal was, but it seems like the reality is that they rarely, if ever are. I think it's pretty awesome that you're able to self reflect on the part that you played in that relationship ending - sounds like agreements and boundaries were needed between all of you that weren't there - too much, too soon on every front, for everyone, and that the impending explosion could have been navigated differently or even avoided with alternate/hard-won perspectives/experiences. All you can do is take those lessons and move forward.
These things aren't anyone's "fault" though! everyone played a part in this, and no one person can step up and take full responsibility.