A sexual relationship involving three partners, all of whom have sexual contacts with each other. C.f. an open triangle, in which two of the members do NOT have sexual relations with each other.
An obvious attribute of a closed triangle is that at least two of the members must be bisexual or homosexual.
A common sexual fantasy for both men and women is the "closed triangle"; it's appeared as a theme in several novels, including Ernest Hemingway's _The Garden of Eden_. As a literary theme, however, it nearly always ends sadly, perhaps because even in their fantasies writers are bound by plausible expectations.