As far as polyamory goes, I have been really 'fortunate' you could say, in that in all my relationships I was effectively the sole girlfriend and so was never confronted with jealousy head on. Mr Oldman has a girlfriend, but she lives in a foreign country so it didn't really affect me on a day-to-day basis. I tried my best to get Mr Wrong a girlfriend, to be a good wingman, but it hasn't panned out... at least not yet!
Then, Mr Oldman went on a couple of dates with one of my good friends. For some reason I want to give this friend the nickname "Miss Prism" after the character from The Importance of Being Earnest. I know she'll object to this, but I like it.
When they were on their date, I was spending the evening feeling super
excited, wanting to know how well it was going, trying to imagine what
was happening. The next morning I eagerly texted them both for details, and then I found out that Mr Oldman and Miss Prism both had a most delightful evening.
I felt jealous. I hadn't felt it before - at least for a long time. It was like a small thing squeezing my heart. It felt weird. I knew that this sort of thing is common and expected, even in poly relationships, so I tried to set it aside.
A few days later, I was visiting Mr Oldman in Perth and I asked him to tell me how the date went in detail. He gave me details from the dinner he cooked for Miss Prism, and even included some of the more saucy ones. As he told me the story of their date, that little squeezing feeling of jealousy evaporated so very quickly. Instead of it being an abstract feeling that he slept with somebody else, instead I had this beautiful feeling that he had this wonderful date with a great person. I had compersion in a big way. Let me be the first to say that compersion is AMAZING, almost on the level of new relationship energy. And the more details I was given, the stronger this feeling was.
I know details aren't for everyone. Mr Wrong hates me going into sexy details, but I think that's because I talk about my sex life so much that he knows he's going to hear me telling 3 or one of my other friends about it in short order anyway. But hearing Mr Oldman's sexy details made me feel so happy that I was glad he was willing to share :).