From Wiktionary, the free dictionary
menage-a-trois (plural menages-a-trois)
- Alternative form of ménage à trois
2011 October 6, Caroline Davies, “Woman jailed after using drill to sink estranged husband's £75,000 yacht”, in The Guardian[1], →ISSN:Fleming had gone to the yacht for a "menage-a-trois" with her then lover, David Brown, and his wife Nemone, the court heard.