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.
Yes, Three Sisters is in four acts. This three-act thing is just another way of thinking about the structure.
Act I
It's our lucky day—part one of the general three-act breakdown also fits with the first act of the play itself. In this act, the sisters are planning for the future and still firmly intend to go back to Moscow. Andrey proposes to Natasha. People come in and out. Vershinin arrives—a breath of fresh air for all the sisters, but particularly for Masha. Things look kind of hopeful, if for no other reason than we can tell that this act is a break in the tedium of their lives before the curtain rose.
Act II
The biggest chunk of the play, the metaphorical Act II covers Chekhov's literal Act II, Act III, and the beginning of Act IV. Natasha begins taking over the house and the sisters are pushed upstairs. Andrey gambles away the assets that might have gotten them back to Moscow. There are fires, drunken doctors, and angry suitors. Things are pretty bleak, and the sisters are as far away from their goals of self-fulfillment as they could be.
Act III
This is where it gets weird: Act III (our plot scheme) = Act IV (à la Chekhov). The revelation of Tuzenbach's death ends Irina's hopes, which she'd already considerably scaled back once the whole true-love-and-fun-job stuff turned out to be pipe dreams. Olga, Masha, and Irina rally at the end of the play, looking with resignation—but also resolve—toward their future. They just have to keep keeping on.