Iago's Manipulation
In the beginning of the act, Iago creates hypothetical scenarios which explain Desdemona and Cassio's actions which don't involve them cheating. However, they are ridiculous. For example, he suggests that they could just be lying in bed together without doing anything. This only serves to strengthen the notion in Othello's mind.
Iago "Proves" the Affair
Iago instructs Othello to hide while Cassio meets him. Cassio and Iago engage in a discussion about Bianca, but Iago brings up Desdemona at one point. By saying Bianca's name quietly so Othello cannot hear it and saying Desdemona's name loudly, Iago manages to make Othello believe that the discussion is about Desdemona. A beneficial coincidence occurs during the discussion for Iago though, as Bianca actually shows up with the handkerchief, having found it in Cassio's quarters. It proves to Othello that Cassio has been sleeping around a lot and angers him because Desdemona has not only been careless with his handkerchief, but this prostitute is now holding it.
Othello Plans to Kill the Cheaters
Othello now thoroughly convinced that he is being cheated on, he plans to get some poison to kill her with. Iago recommends that he instead strangle her. Iago only recommends this because it will be almost certain that Othello will get in trouble for the murder instead of having a chance to get out free.
The Serpent's Curse
The Serpent’s Curse is the curse that God gave to the snake in the story of Adam and Eve for deceiving Eve into taking a bite from the apple. Emilia puts the curse on whoever is maliciously placing Othello’s incorrect thoughts about Desdemona into his head, without actually having any idea that it is her own husband. This causes some things to start to go poorly for Iago in Act 4 Scene 2. After Desdemona and Emilia finish their conversation with Iago and leave, Iago is attacked by Roderigo. Roderigo is angry because what Iago says never comes to fruition, but Iago does manage to talk his way out of injury due to the sheer foolishness of Roderigo. Also, Roderigo says that he will go to get his jewels back from Desdemona, but if she will not give them back he will seek compensation from Iago.
Emilia on Cheating
Emilia says that it is "husbands' faults if wives do fall," (Scene 3) essentially meaning that it is a husband's fault if their wife cheats on them because it would mean that the husband hasn't done enough to keep her happy or treated her poorly.
Foreshadowing
Likely the strongest example of foreshadowing in Scene 3 is after Emilia lays the sheets that Desdemona asked her to on the bed. Desdemona says “If I do die before thee, pr’ythee, shroud me in one of those same sheets.” This is the strongest example of foreshadowing in the scene because we know of Othello’s plan to kill Desdemona tonight, which just so happens to be the night those sheets are on the bed, and she is talking about what she wants Emilia to do if she dies before her.
The Devil and Hell in Act 4
“Naked in bed, Iago, and not mean harm! It is hypocrisy against the devil: they that mean virtuously and yet do so, the devil their virtue tempts, and they tempt heaven” (Othello, Scene 1).
“It is not words that shake me thus: pish! Noses, ears, and lips. Is’t possible? Confess, handkerchief! O devil” (Othello, Scene 1)!
“Let the devil and his dam haunt you” (Bianca, Scene 1)!
“O devil, devil! If that the earth could teem with woman’s tears, each drop she falls would prove a crocodile. Out of my sight” (Othello, Scene 1)!
“Come, swear it, damn thyself; lest, being like one of heaven, the devils themselves should fear to seize thee: therefore be double-damn’d. Swear thou art honest” (Othello, Scene 2).
“O, ‘tis the spite of hell, the fiend’s arch-mock, to lip a wanton in a secure couch, and to suppose her chaste” (Iago, Scene 1)!
“Heaven truly knows that thou art false as hell” (Othello, Scene 2).
“Patience, thou young and rose-lipp’d cherubin, ay, there, look grim as hell” (Othello, Scene 2)!
“You, mistress, that have the office opposite to Saint Peter, and keep the gate of hell” (Othello, Scene 2)!
“A halter pardon him! And hell gnaw his bones” (Emilia, Scene 2)!
“It is not words that shake me thus: pish! Noses, ears, and lips. Is’t possible? Confess, handkerchief! O devil” (Othello, Scene 1)!
“Let the devil and his dam haunt you” (Bianca, Scene 1)!
“O devil, devil! If that the earth could teem with woman’s tears, each drop she falls would prove a crocodile. Out of my sight” (Othello, Scene 1)!
“Come, swear it, damn thyself; lest, being like one of heaven, the devils themselves should fear to seize thee: therefore be double-damn’d. Swear thou art honest” (Othello, Scene 2).
“O, ‘tis the spite of hell, the fiend’s arch-mock, to lip a wanton in a secure couch, and to suppose her chaste” (Iago, Scene 1)!
“Heaven truly knows that thou art false as hell” (Othello, Scene 2).
“Patience, thou young and rose-lipp’d cherubin, ay, there, look grim as hell” (Othello, Scene 2)!
“You, mistress, that have the office opposite to Saint Peter, and keep the gate of hell” (Othello, Scene 2)!
“A halter pardon him! And hell gnaw his bones” (Emilia, Scene 2)!