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
D’Artagnan arrives in Paris with not much more than the big dreams of becoming a Musketeer.
Act II
D’Artagnan becomes best friends with three Musketeers, and together the four of them have all sorts of glorious adventures.
Act III
D’Artagnan becomes a lieutenant in the Musketeers.