two is company, three is a crowd
Proverb meaning that a pair can get along well, but adding a third person often leads to conflict or discord.
As the proverb goes, 'Two is company, three is a crowd,' and indeed, traveling as a trio often leads to arguments.
A Japanese proverb contrasting the harmony of a pair with the discord that often arises when a third person joins. The exact origin is uncertain, but it reflects a common folk observation about group dynamics.