Act 3 represents a significant turning point in the play Othello, as Iago's manipulations bring out Othello's jealousy. Already, even before this point in the play, we have see his plan beginning to take shape, as he masterminds Cassio's disgrace. However, act 3 is where you truly begin to see Iago's machinations come together as Othello is launched on the path to destruction.

With that in mind, in the context of your assignment, I would suggest that this act represents the critical hinge on which this pattern (described in your assignment) turns: this is the moment where Othello loses his grip on reason. Iago plants a seed of doubt in Othello's mind concerning Desdemona's faithfulness to him, and Othello proceeds to nourish it, even as Iago continues to feed Othello's suspicions with his plotting. But it is Othello who chooses to fixate on those suspicions, turning abusive against his wife. By the play's ending, he will become so swept up in his jealousy that he murders Desdemona, only moments before her innocence is established.

Act 3 is thus critical in setting Othello down this path, entrapping him within Iago's lies. Already, you can observe his abusive side manifesting in his interactions with his wife, a side to his personality that will remain in force, with his behavior eventually escalating into murder.