What is the term for a literary work composed of three parts, each of which is a complete work itself?

A trilogy is the correct term for a literary work that is divided into three parts, where each part stands alone as a complete work. This structure allows for a continuous narrative that can explore complex themes, character arcs, and plot developments over the course of the three works while still providing a satisfying experience for readers in each individual installment. Literary trilogies often build upon each other's themes and storylines, making the connection between the parts significant to the overall understanding of the narrative.

Triptych, while it refers to a work of art or literature that consists of three related panels or sections, is not typically used to describe a series of literary works. The other terms—trio and triolette—do not apply to a series of three literary parts; a trio refers to a group of three, and a triolette is a specific type of poem or a three-line poetic form, which does not encompass the idea of an expanded narrative like a trilogy does.