Threesome?

Catalina • Insta: cat.astrophe.x 👻: catd7
So my boyfriend and I are thinking of having a threesome with another female. Well, he suggested maybe with his ex, as she's very very curious person when to sex stuff. I've met her and I've seen her and him interact, and while it's obvious neither really have feelings for each other anymore, the thought of doing it with her makes me feel a little uneasy. He completely respects my decision of not doing it with her, but it's just neither of us can think of anyone else who would possibly even want to have a threesome. He also suggested maybe if we did it with her, that it could be more centered around me instead of him. Any thoughts on the situation?