My background: I was in a long term relationship with a couple. I loved them both, but was only sexually involved with him. The other female and I had interests outside of her husband and spent lots of time together on our mutual interests. We called ourselves a triad despite the lack of a sexual component between myself and the other female. It was a beautiful experience. It lasted a couple of decades, until he suddenly passed away.
Fast forward: thought I was entering into the same type of dynamic. It was great until the other female who had been promoting a marriage ceremony for the 3 of us, suddenly did a 180 with no explanation. Her husband kept seeking the root of her feeling with the idea that whatever it was could be addressed. My gut feeling was that whatever the cause, it couldn't be fixed. I lived in limbo for months as the husband tried to understand and allay her fears. It was agonizing. Eventually, she too gave him an ultimatum which would deny the husband and I any kind of a friendship. A year after I left their lives, they separated and are currently divorcing.
Due to my last experience, my instinct is to tell you to listen to your gut and run. However, you seem more adept at drawing boundaries than I was, and so perhaps it is not necessary to give up yet. However, here is the question I would ask before proceeding if you have not already asked it: what has changed that makes them think they are capable of behaving differently than before? The fact they love you is not enough. I was loved too. It made me stay far longer than I should have. It didn't change the outcome.