An informal way to express a situation when two people desire privacy and a third one is present. - A proverb. * /Beth and Carl wanted to be alone so when Maggie joined them they said, "Two's company; three's a crowd."/
TWO'S COMPANY; THREE'S A CROWD
Meaning of TWO'S COMPANY; THREE'S A CROWD in English
Dictionary of English idioms . Словарь английских идиом. 2012