The third of three, or a group of three.
The Bartimaeus Trilogy is a series of books written by Jonathan Stroud. They consist of The Amulet of Samarkand, The Golem's Eye, and Ptolemy's Gate. They revolve around the title character,...
'The Divine Comedy' or 'La Divina Commedia', made up of three volumes: 'Inferno' (hell), 'Purgatorio' (Purgatory), 'Paradiso' (Paradise / Heaven).
Oedipus Rex, Oedipus at Colonus, and Antigone
I read and enjoyed all the books in The Lord Of The Rings trilogy
Untrilogical i.e. a group of any number except three (e.g. "John Avildsen decided to make the Rocky movie series untriloical with Rocky 4").