Excerpt
Trilogies
Trilogies use a very different structure than series books. There are plenty of series that have three volumes that have been mislabeled as trilogies just because of the number, but true trilogies are one story told through three books that go together.
Trilogies are typically written as follows. Book one, the writer presents the protagonist and other characters, setting, and story problem. Book two is to add to the protagonist's knowledge, add friends, and up the stakes. But, the bad guy is still winning until the end of the book, where the good guys get some knowledge that bolsters their confidence. Book three shows the protagonist's renewed faith, and the bad guys are defeated.
While you can read them out of order and alone, they are meant to go together. Often times, while they follow Hero's Journey, a firm seven-point structure is not there. Sometimes they even end on a cliffhanger. Some people say that the "father" of all trilogies is The Lord of the Rings. Tolkien originally planned on publishing the story as one big book, but the printers couldn't bind a book that big so the story was split into three parts and published as three books instead.
Trilogy examples are, of course, Lord of the Rings, Reconners by Brandon Sanderson, Lisa Mangum's Hourglass Door, and Charlie Holmberg's Paper Magician. Although the movie preceded the books by a long shot, a great movie example is the first three Star Wars: A New Hope, Empire Strikes Back, and Return of the Jedi.