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.
This follows the structure of the novel very closely—Hemingway himself divided it into three "Books."
Act I
The cast of characters is introduced. Jake and the crew hang out in Paris. Cohn struggles with Frances; Jake and Brett discuss their relationship. Everyone is ready for a vacation, so they plan a trip to Spain.
Act II
Act II concludes with the end of the fiesta. Exhausted, miserable, and uncertain, the gang parts ways. Cohn leaves earlier, after attacking just about everyone else, Brett runs off with Romero, and the three men, Jake, Bill, and Mike, are left to console each other. As Jake plans for some alone time to recover in the relaxing natural setting of San Sebastian, we are left unsure of how everyone’s relationships will pan out.
Act III
In the novel’s last chapter, Jake is called away from his peaceful recuperation by the seaside by Brett, who needs him once again. The book concludes with a final emphasis on the impossibility of their relationship—all avenues to happiness appear to have been closed. The novel ends on a wistful but negative note, with Jake rejecting the possibility that he and Brett could ever have been together.