For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
In Steppenwolf, Act I lasts from Harry's arrival in the town to when he meets Hermine and she tells him that he must a) love her and b) kill her. Yowch.
Act II
The preparation for the costume ball (with Harry learning to loosen up get a little rhythm in his step) and, finally, his killing Hermine in the Magic Theater all make up crazy Act II.
Act III
Act III is short and sweet. Harry realizes that he's missed the point of life and resolves to try to learn to laugh. It's not as cheezeball as it sounds.