What Was Once What We Did Is No Longer What I Want To Do

In college I lived in a threesome consisting of my female roommate and a male grad student and me. It was all consensual, sharing the same space, the same bed. Although she and I weren't 'lovers' during sex we did love on each other. As time went on he became superfluous and we moved on without him.

That was fifteen years ago. She met a guy at work, and now he is in our bed. And I do not like it.

I want her back, exclusively. Sex with him is horrid.