Act IIIs are usually the turning points in Shakespeare's tragedies. Here are the key shifts:
- Here, Macbeth goes even further toward the dark side: he killed his king in Act II, and now he kills his best friend Banquo.
- Also, Banquo (Macbeth's foil) will become his doppelganger and literally haunt him. He is a revenge ghost, another element of the supernatural.
- We begin to see the gender differences emerging. Macbeth separates from Lady Macbeth (she does not plan Banquo's murder).
- And, we have the foreshadowing of madness evident, particularly with Macbeth, even though Lady Macbeth will be afflicted more seveley.
Verbal Irony: Macbeth says, "I wish your horses swift and sure of foot; And so I do commend you to their backs. Farewell."
Death/Sleep Imagery: Macbeth says, "In the affliction of these terrible dreams
That shake us nightly: better be with the dead,
Whom we, to gain our peace, have sent to peace,
Than on the torture of the mind to lie
In restless ecstasy. Duncan is in his grave;
After life's fitful fever he sleeps well;
Treason has done his worst: nor steel, nor poison,
Malice domestic, foreign levy, nothing,
Can touch him further.
Blood / Death Imagery:
Blood hath been shed ere now, i' the olden time,
Ere human statute purged the gentle weal;
Ay, and since too, murders have been perform'd
Too terrible for the ear: the times have been,
That, when the brains were out, the man would die,
And there an end; but now they rise again,
With twenty mortal murders on their crowns,
And push us from our stools: this is more strange
Than such a murder is.
The above post is very good, and I'd like to add three more points to it. These points are soliloquy, illusion and allusion.
First, come to the soliloquies. In the first scene of the act in Macbeth, we find Banquo uttering to himself:
"Thou hast it now, King, Cawdor, Glamis, all,
As the Weird Women promised, and I fear
Thou plyed'st most foully for't..."
These lines show that suspicion is being arisen and condensed in Banquo's mind regarding Macbeth's devilish deeds. He, further, asks to himself some rhetorical questions:
"If there come truth from them,
As upon thee, Macbeth, their speeches shine,
Why by the verities on thee made good
May they not be my oracles as well,
And set me up in hope?"
And, these prove that, Banquo who was once indifferent to the witches' prophecies, has begun believing them. Surely, this paves way to a turning point in the character of Banquo. Besides, through Macbeth's famous soliloquy, "To be thus is nothing,/ But to be safely thus...", Macbeth's evil intention to finish the threats to his way - Banquo and his progeny - becomes more obvious.
Then comes the point illusion. In this act, here is a reference to visual illusion. In the fourth scene, Macbeth hallucinates Banquo's ghost during the feast which is an outcome of his inner stress and fear.
Allusion is another literary element used in act 3. In the first scene, When Macbeth soliloquises regarding his intention to kill Banquo, he alludes to Octavius Caesar (Augustus Caesar):
"My genius is rebuked, as it is said
Mark Antony's was by Caesar."
And, lastly I'll add an additional example of verbal irony which is quite important. Hecate, in her speech in the fifth scene, ironically utters: "security is/ Mortal's chiefest enemy", which is an indication to Macbeth. This ironical statement has two meanings. First, it means that, Macbeth, who is blindly driven by the prophecies of the witches, thinks himself secure for ever, and this over-confidence would make him more endangered. Secondly, Macbeth crazily wants to be assured of the throne and keep his power safe and secure at any cost; and, this obsession would bring more perils for him.