Two is company, three's a crowd
two's company(, three's a crowd)
A third person would make a group of people awkward or uncomfortable, especially when the other two are lovers or close friends. I was worried when the new boy moved into the neighborhood and started hanging out with John and his best friend—two's company, but three's a crowd.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
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.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.