Trilogy

tril′ō-ji, n. the name given by the Greeks to a group of three tragedies, either connected by a common subject or each representing a distinct story—the Oresteia of Æschylus, which embraces the Agamemnon , the Chœphorœ , and the Eumenides . [Gr. trilogia — tri , tris , thrice, logia , speech— legein , to say.]