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
After dumping Sibyl and dealing (quickly) with her death, Dorian reads the yellow book for the first time, and is profoundly changed… for the worse.
Act II
Dorian kills Basil in a fit of moral-crisis-driven rage, and blackmails Alan Campbell into destroying the evidence.
Act III
Dorian's close call with James Vane makes him reevaluate his life—should he in fact be trying to reform himself? He briefly thinks he can be good again, but, alas, he can't. In an attempt to get rid of the evidence of his sins, he slashes the portrait, and dies.