*trilogy*

n. [ Gr. trilogi`a; pref. tri- (see Tri-) + lo`gos speech, discourse: cf. F. trilogie. ] A series of three dramas which, although each of them is in one sense complete, have a close mutual relation, and form one historical and poetical picture. Shakespeare's “ Henry VI.” is an example. [ 1913 Webster ]

On the Greek stage, a drama, or acted story, consisted in reality of three dramas, called together a trilogy, and performed consecutively in the course of one day. Coleridge. [ 1913 Webster ]