They were getting divorced whether they "opened" up their marriage or not.
Amen.
A second cousin of mine is involved in a throuple. I'd never heard of this before it was brought up around the dining room island when I was deep into the wine one year at the family Xmas get-together. He was in a relationship with two girls at that time. I nearly spit my wine out in surprise. So you have to dote on two different women and all that AND there weren't threesomes?!?! I feel like you're maybe missing the best part. However, he has now since gotten involved with a throuple with another man as the third. They live in separate houses on the same property and there is a child involved. I have no fucking clue which of them is the biological father.
Hey, I'm no kink-shamer. If that works for you, by all means go and have a good time. I just know personally I could never make it work on my end -- unless there were threesomes (not the Devil's variety either /wink).