Orangesmartie is insightful to consider "re-wrapping" that package. And bookbug is spot-on about not being willing to give up the best thing you've ever had just to fit into societal norms. We all struggle with this from time to time, as it's just not possible for most of us to be out to everyone we know. You're not alone.
I'm on the opposite side of the triangle - the wife in an 18 year marriage that opened up later. Five years ago we met a woman who was in your role. While we invited her into our lives, she was programmed by society to think she had to have that white picket fence dream. We've since moved on to others in different configurations, but still keep a door open for when she walks back in, as she inevitably does from time to time.
If you do close your triad, that is considered polyfidelitious, meaning closed to other partners. However, I do agree that just as your triad "happened," sometimes love finds you when and where you least expect it. If it's meant to be, it will hit you over the head, even if you're not keeping your eyes open for it.
Best wishes for navigating this love with no field guide. Talk, trust, and talk some more.