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
The first act sets up the rules of this future society—the way these people live and the world they have made—as well as the basic threads of conflict. In this case, it starts with Record 1 and lasts just about to the executions in Record 9. After that, the wheels start falling off of his wagon a little bit.
Act II
Act II runs from Record 10 (where he gets funky with I-330 and notices himself "dividing" into two people) until Record 25 (when MEPHI stages its dissent at the election and the revolution begins in earnest. It covers D-503's slow emergence from inhuman robot to feeling human being, as well as revealing what MEPHI is and what they want with him.
Act III
Act III details the rest of the book. As the rebellion spreads D-503 ultimately faces the music for his perceived defiance of the State. It covers D-503's final contact (and embrace of) his humanity, just before the State wipes it out for good. A pity that the rebellion he enabled won't be killed so easily.