Threesome

(Design for Living, Ernst Lubitsch 1933)

(Les Valseuses, Bertrand Blier 1974)

(Les nuits fauves, Cyril Collard 1992)

1+1=3

Not in Database:
Willie & Phil (1980)
Swingers (Netherlands, 2002)

Also take a look at Ally’s list with a similar topic: http://mubi.com/lists/9089