Two is company, three’s a crowd
A way of asking a third person to leave because you want to be alone with
someone.
(Often implies that you want to be alone with the person because you are
romantically interested in him or her).
When Lucy followed Mark and Nora into the drawing room, Nora
turned to her and said, “Two’s company, but three’s a crowd.”
(From The Free Dictionary by Farlex)
