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
Disclaimer: Jazz doesn't work in a linear fashion. What constitutes a first act—a.k.a. the beginning of the text to the point of no return—doesn't happen at the beginning of this book. Nope, history shows up in all sorts of flashbacks throughout.
But working in a linear fashion, our Act I is everything that happens prior to Joe seducing Dorcas. Think: Vera Louise getting knocked up and moving to Baltimore with True Belle, Golden Gray finding Wild and meeting his father, Rose Dear's suicide, and the lives of Violet and Joe up until October, 1925. This is all contained in the Act I of Jazz.
Act II
Again, Act II is shredded into confetti and strewn all over this non-linear narrative. But that facts that make up an Act II, from the point of no return through the bleakest of the bleak times, are the events from October 1925 to January 1926.
We see the entirety of Joe and Dorcas's affair, their breakup, Joe's breakdown, Dorcas getting together with Acton, Joe shooting Dorcas, and Violet attempting to slash dead Dorcas's face. All the really ugly stuff in the Joe/ Violet/ Dorcas triangle happens here, as do the first depressing weeks after the funeral when Violet is raging and Joe is comatose with grief.
Act III
Ah, the resolution. It's scattered like eggs at an Easter hunt, but it's all there for the finding. This act begins with Violet talking to Alice. Violet reconnects with the memory of her sweet grandmother True Belle, and starts to think of Dorcas as less of a man-eater and more of a daughter-figure. Felice shows up and spreads the verbal equivalent of Neosporin on everyone's emotional rashes—Joe is roused back to life and happiness, Violet has a daughter-figure, and eventually Joe and Violet's relationship is healed. Peace restored; curtain falls; the end.