Trilogy (noun)

1. A group of three literary or dramatic works that are connected and can be seen either as a single work or as three individual works.
2. A series of three related works, events, or ideas.

Origin:

From late latin trilogia, from greek trilogia, from treis ("three") + logos ("speec.h")

Examples:

1. The Lord of the Rings is a famous trilogy of novels.
2. The Star Wars trilogy is a set of three movies released in the late 1970s and early 1980s.
3. The Harry Potter series is a seven-book trilogy.
4. The trilogy of plays by Shakespeare, Hamlet, Othello, and King Lear is known as the tragedies.
5. The trilogy of the American West, The Big Sky, The Way West and Fair Land, Fair Land is a set of three novels by A.B. Guthrie Jr.