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.
No, you're not crazy. The Seagull is in four acts. This Three-Act structure is just another way of breaking down the events in the play.
Act 1
"Act 1" encompasses Chekhov's Act 1 and part of Act 2, up until Konstantin brings in the dead seagull. Konstantin seeks success and the love of Nina—to him, two inseparable goals—and bangs his head against the wall trying to secure both of those.
Act 2
The rest of Act 2, all of Act 3 and the beginning of Act 4. Rejected by Nina when Trigorin consumes her, Konstantin still can't give her up. He attempts suicide, and welcomes her back for a visit.
Act 3
The rest of Chekhov's Act 4, after Nina's departure. Bereft of Nina, facing a house of unfriendly family and neighbors, Konstantin kills himself.