Book 3 is the Climax.
In book 3, the hero rises up to the occasion. He conquers the enemy, overcome his weaknesses, and figures everything out.
The hero is depressed, but he will soon overcome his shortcomings to strike back.
He finds a way to overcome the enemy in a way that wasn’t possible before.
In book 3 the hero needs to become the person he is destined to be.
EXAMPLE: In The Girl of Fire and Thorns by Rae Carson, Elisa needs to become queen, take over the kingdom, the leadership and crush the enemy. A battle needs to be fought to get her there.
In book 3, there are no new elements introduced. No new subplots and no new characters.
This is where you need to tie as many loose ends as you can.
The hero faces the antagonist in one last decisive battle.
The hero gathers his allies around and train for the battle. The hero now knows who he is and where he is supposed to go. He makes preparations for the battle.
The plan is unexpected and will ask a lot of resources.
EXAMPLE: In Twilight, Bella gathers people and face the Volturi in battle. She can face them because she is vampire now and she has special powers that will overcome the Volturi defenses.
EXAMPLE: In The Immortals, in book 5, Ever and Damen train and prepare for the final battle with Haven.
Johnson, Elana. "Writing a Trilogy, Part Three: Book Three."