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
Samson sits in prison contemplating and discussing his terrible situation. It's even more depressing than it sounds.
Act II
But it gets worse, when Samson receives visits from his father, his ex-wife Dalila, and a hero of the enemy, none of which cheer him up at all. In fact, these visits just make him more angry and isolated. Finally, a messenger arrives ordering him to appear at the theater. Samson refuses and the messenger stomps out.
Act III
For unknown reasons (we're guessing God), Samson has a sudden change of heart and agrees to appear at the theater. And boy does he! While leaning oh-so-casually against the pillars, he collapses the building, killing himself and, as a bonus, all the rich Philistines who are watching.