My wife and I lived with our third in just your situation for ~5 years and it was glorious and wonderful. We all lived in the same rural yard and spent as much time together as possible. Eventually he met and fell in love with a woman that was adamantly mono and he was forced to choose. We told him we loved him and wanted him to be happy so do what he felt was right for himself and let's remain friends. Sadly they moved across the continent and our happy little triad became a couple again. A very happy, content couple but a couple nonetheless. We would love to become a triad (or a quad) again and, while winning the lottery twice in a lifetime is unlikely, we keep our hearts, minds and arms open hoping to meet someone special again.
Good luck and happy loving.