There are two types of trilogies or sequels: the stand-alone books (Harry Potter, The Magic Tree House, The princess Academy) and the books that are an essential part of the whole (Lord of the Rings, The Hunger Games, Mortal Instruments).
Most of the time MG books are stand-alone series while YA novels build on each other. That’s because many MG books are based on an adventure (plot driven) while YA books are based on psychology, on the protagonist’s internal journey (character driven).
The stand-alone novel has a crisis that is solved at the end of the book. It is often a journey with a finish line, and the hero returns to the start at the end of the adventure. Rarely the hero evolves along the way or grows up as in Harry Potter, but each book may follow a year in high school.
The sequels do not always come in three, but they usually follow the three-act pattern that makes up the traditional novel.
Harry Potter went to nine, Sookie Sackhouse counts seven books, the Song of Fire and Ice is up to seven books.
STRUCTURE OF THE TRILOGY:
A trilogy where books build on each other requires huge steps toward the resolution: usually the setup, the rising action and the climax. Some series have an extra book for the resolution. Some have 2 or 3 books for the rising action.
This means that in any given 400-page novel, Act One would take about a 100 pages long. If we think in term of books, book 1 is Act I; book 2, 3 are Act Two and book 4 will constitute Act Three.
To keep it simple, the three books of the trilogy constitute the three traditional acts in a stand-alone book. It is of utter importance therefore that you know the final ending in order to be able to plot the trilogy. Also, if you have a series containing five novels, then consider telling a story in five acts.
STRUCTURE IN DETAILS:
Often, book 1 presents the hero’s personal/intimate journey or problem and how the hero embraces or is thrown into the adventure.
In book 2, the hero often confronts the political order (Divergent by Veronica Roth), a master/king (The selection by Kiera Cass), a world order (The Maze Runner by James Dahner), or an empire (Mistborn by Brandon Sanderson). The hero keeps trying to solve his problems, but fails. However, he becomes a hero during his trials.
Book 3 often shows the hero breaking down the political order or changing the way the world functions. It’s the Climax and Resolution. The hero often becomes a leader and gathers allies around him for the last battle.
During the trilogy, the order of events often goes from personal, to the resistance, to the break down the world established. That’s because the stakes go higher and higher as the series progresses. The hero becomes more involved with the external world and often that implies saving oneself, one’s family, then a group, then the world.
EXAMPLE: The hero goes through metamorphosis. In Twilight by Stefanie Meyer, part 1 shows Bella changing from the awkward naïve and innocent girl to a woman who can take her destiny into her hands. In part 2, she defies the Volturi order and saves Edward from a certain death. She’s become a hero. In part 3, she turns into a vampire and leads part of the vampire world. She goes through a complete metamorphosis.
BIRTH OF A TRILOGY:
Sometimes an author would write his debut novel, then wait to see how it fares, then decides later to write a trilogy. Trilogies are not always popular, but it is some kind of insurance for the author if a contract is signed.
Some readers get frustrated by trilogies because there is a long delay between the books. Readers do not appreciate the cliff hanger at the end. Some readers even wait until the whole series has been release before they start reading them, which is not helpful for an author who is waiting to see how his first novel is received.
“Leigh Bardugo: When I started writing Shadow and Bone (2012), my only goal was to finish the book. I had no thoughts of a trilogy. But about halfway through that first draft, I began to think of a larger arc for the story and to take notes for a potential second and third book. At the time, the idea that anyone would want to buy one book from me—let alone three—seemed pretty farfetched. When Jo and I first talked on the phone and she offered me representation, I was relieved to hear her say that Shadow and Bone felt like the start to a trilogy.”
Quote from: http://www.booklistreader.com/2015/02/24/books-and-authors/publishing-u-a-single-novel-a-trilogy-a-series/