three's a crowd - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Also, two's company, three's a crowd. A third person spoils the ideal combination of a couple, as in No, I won't join you—three's a crowd. This expression, alluding to a third person spoiling the privacy of a pair of lovers, was already a proverb in 1546. For a synonym, see fifth wheel.