ménage à trois
Concise Oxford English Dictionary © 2008 Oxford University Press:
ménage à trois /meɪˌnɑːʒ ɑː ˈtrwʌ/
▶noun (pl. ménages à trois pronunc. same) an arrangement in which a married couple and the lover of one of them live together.
– origin Fr., ‘household of three’.
'ménage à trois' also found in these Oxford entries:

