“The Threesome” is quick to establish its rom-com bona fides by opening at a wedding, but like much in Chad Hartigan’s sixth feature, there’s a little something more going on when it sets the stage for a marriage of sensibilities behind the scenes. The director, who is capable of both contemplative dramas (“This Is Martin Bonner”) and raucous comedies (“Morris From America”), tries to strike a balance between screenwriter Ethan Ogilby’s cringe-humor premise of an impromptu ménage à trois and the more serious relationship dynamics it suggests. The results are mixed in ways the filmmakers probably didn’t intend, but they’re at once genuinely intriguing and enormously charming given the talent involved.

Those taking the vows are friends of Connor (Jonah Hauer-King), who has hoped for some time to be reciting some of his own with Olivia (Zoey Deutch), a waitress who has long put off...