What are a few important literary devices in act 3 of Macbeth, and how do they establish themes?
There are more literary devices than I can name, but in Act 3, Scene 1, there are three that stand out: alliteration, metaphor, and allusion. These all relate to the theme of betrayal.
The first literary device that stands out is in Act 3, Scene 1, when Banquo comments to himself on Macbeth’s succession to king. He shares his suspicions with the audience.
Thou play'dst most foully for't: (p. 40)
The repetition of initial consonant sounds is called alliteration. In this case, the alliteration of the f sound at the beginning of “fear,” “foully” and “for’t.” This reinforces the idea that Banquo is frustrated and angry. The bitterness comes out through the alliteration.
Banquo also uses an interesting metaphor when talking to Macbeth.
I must become a borrower of the night
For a dark hour or twain.(30) (p. 41)
This is a metaphor because of course you cannot borrow night. It is also an interesting element of foreshadowing, because Banquo is about to die.
Finally, there are allusions to historical events. An allusion is a reference to something that happened in the past. In this case, there is an allusion to another famous betrayal: Ceasar and Mark Antony.
My genius is rebuked, as it is said(60)
Mark Antony's was by Caesar.
As Caesar thought that Mark Antony betrayed him, Macbeth thinks that Banquo betrayed him. This connects again to the theme of betrayal present so highly in this act.