Realism
Chekhov believed that theater should reflect life. So just think of his playwriting as the opposite of Hollywood. He doesn't give us a potboiler with lots of onstage sex, violence, and suspense. Events like the fire, the sexy sleigh ride, the baron's death, Andrey's devastating gambling match—we don't see any of them. Instead, they're communicated through after-the-fact reports and discussed by the characters now dealing with the repercussions. (This is the way Greek playwrights did it too, by the way.)
What does this mean for us? Well, things happen in The Three Sisters like they do in real life. Can we get a duh in here? Anyway, just like in life, some people die, some are ruined, some have affairs. We're not often witnesses to the tragedies and dramas themselves, but sit us down at a dinner table, or pour us a strong marg, and we're ready to dish it out.