I was in a threesome for over a year, it was with all mutuare adults, I was the odd man into a marriage called as we know it as a ménage à trois , it was all good until the wife started to get weird and writing notes about how her and I could go off one our own without the husband. At that point it had to sadly end, it was the best experience I had in my life to date and that was 30 years ago. I don’t have Any regrets. So my advice is go into it with open eyes and don’t be afraid to call it if it needs to end. Keep thinking with the right head, as they say.
Enjoy and wish the best for you