Most good stories start with a fundamental list of ingredients: the initial situation, conflict, complication, climax, suspense, denouement, and conclusion. Great writers sometimes shake up the recipe and add some spice.
Initial Situation
The sisters celebrate Irina's birthday and fantasize about returning to Moscow.
Three Sisters opens with a lot of possibility. Irina holds out hope not only for Moscow, but for finding fulfillment through work. Olga isn't overflowing with optimism—she's always kind of stuck in the past—but she applies herself to mothering the clan and can join in Irina's enthusiasm. As for Masha, she's pretty pissed with the whole order of things, but things start to look up when Vershinin arrives—he's just the sort of intellectual presence she needs to shake up her boring lifestyle.
In other news, Andrey and Natasha are about to get engaged. Also, the doctor gives Irina embarrassingly big birthday presents, the baron tries to flirt with her, Solyony is obnoxious, and so is Masha's husband, the "pedagogue." Still, nothing too dismal is happening yet. All in all, it looks like pretty good stuff is happening for the Prozorovs.
Conflict
Natasha begins to conquer the house by turning away the carnival party, and things keep going downhill from there.
Uh oh. It becomes clear that those Act I dreams may not come to pass after all. It's only been two years, but Natasha and Andrey have already fallen out of whatever love thang they once had going on. Natasha's having an affair and is planning to kick Irina out of her room to make baby Bobik more comfortable. Since when do babies need that much space? Clearly, the house is in a spot of disarray.
As for the sisters themselves, Irina's exhausted by her unfulfilling job at the telegraph office. Olga's been filling in for the headmistress despite being really, really not into it. And Masha is knee-deep in an affair with Vershinin, but his whole wife-trying-to-commit-suicide thing doesn't make that too fun either. Things are kind of messed up, hopeless, and complicated—and it's Russia, so you better believe it's about to get worse.
Complication
There's a fire. Run!
The Prozorov house is safe, but that doesn't mean things are easy. The fire keeps the family up all night as they help people fleeing from their burning homes. Olga's gathering clothes for the victims, and one by one the characters come up to her bedroom. The high stress level resulting from the fire basically ups the stakes of all the conflicts that began to emerge in the prior section. Get ready for…
Climax
Olga and Natasha confront each other. Andrey confronts the sisters. (Being non-confrontational is for Americans.)
All the characters are totally tweaked out by the fire, resulting in the most emotionally charged scenes in the play. Olga and Natasha fight over the fate of Anfisa, the old servant woman. Natasha wants the old woman out since she can't work; Olga wants to protect her because she's actually a nice person. This battle is followed by Chebutykin's outburst, Irina's nervous breakdown, and Andrey's tearful apology for mortgaging the house.
Suspense
The soldiers are about to march away, and the baron heads off to duel Solyony.
The baron gives such a sweet and philosophical farewell to Irina that the audience starts to worry about him (like Irina, we were kind of indifferent to him before. Oh, the wonders of philosophy). Anyway, when he asks her to put some coffee on for him, he seems like a goner. Still, we wait in suspense. Will he make it? Will he and Irina escape and get married and be happy together? That's the only hope we're clinging to, since everyone else seems pretty miserable, what with the soldiers heading out (including Masha's lover Vershinin), Olga moving in at the school (no one wants to be a headmistress), and Andrey babysitting while his wife hangs out with her lover (ouch).
Denouement
The baron is dead. The soldiers leave.
Sure, Baron Tuzenbach wasn't the love of Irina's life or anything, but she had come around to the idea of starting a new life with him. She was ready to go for the plunge. After this disappointment, she must figure out what to do with her life all over again.
Meanwhile, Masha's sobbing her head off after Vershinin marches away and Olga's not feeling so great either. This is when we realize the whole "tragedy" thing is about to come to a head.
Conclusion
The sisters resolve to keep on living.
Well, what's the alternative? Olga, Masha, and Irina have all experienced a great deal of disappointment and pain, but hey, they have each other. Olga, in particular, wants to understand why they have to suffer ("If only we knew!"), but no answer seems forthcoming. All they can do is keep working and pushing on into the future. What will happen to these sibs next? We smell a sequel…