Русско-английский словарь пословиц и поговорок
ГДЕ ДВОЕ, ТАМ ТРЕТИЙ ЛИШНИЙ
The third person should leave two people who are having a romantical affair alone
Var.:Двум любо, третий не суйсяCf:Two is a couple, three is a crowd (Am.). Two is company and three is none (Br.). Two is company; three is a crowd (Am.,Br.). Two's company, three's trumpery (Br.)