I have a few good ones, our most recent was actually a woman I dated when I was 16(1990) I ran into her at the grocery store, she and my wife hit it off, we invited her to join our relationship, but that only lasted 6 years.
Then there's our current girlfriend, we met her 15 years ago at a metaphysical shop, her marriage collapsed, she showed an intrest, and things just happened.
Finally, my wife, I met through an ex I was fooling around with at the time, she invited me over for a threesome, which she then fell asleep and made it a twosome just my wife and I, we've been together ever since.
37, M, married, 15 years, in a happy triad for the same amount of time. It's not always easy, but it sure is fun.