Two is company, three's a crowd

Two is company, (but) three's a crowd.

 and Two's company(, three's a crowd).
Prov. 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." Bill: Can I go to lunch with you and Tom? Jane: Two's company, three's a crowd, Bill.
See also: crowd, two