This scene focuses on Hecate's anger at her subordinates for dealing with Macbeth without involving her, their leader, in their arrangements. The most obvious literary effect in her monologue is the rhyme. She speaks in rhyming couplets. The effect is lyrical and ties in with the rhythmic pattern of the language the witches use when they perform their spells.
It is generally believed that charms have to rhyme and one can note this convention in most literature in which spells, charms and curses are being cast. Furthermore, the belief that there is power in words is further emphasized in the manner that Hecate, as well as the other witches, speak throughout the play.
At the beginning of the play, the witches use the same structure:
When shall we three meet again
In thunder, lightning, or in rain?
When the hurlyburly's done,
When the battle's lost and won.
Fair is foul, and foul is fair:
Hover through the fog and filthy air
This is sustained throughout. Note, for example, the first witch's final words to Macbeth:
Ay, sir, all this is so: but why
Stands Macbeth thus amazedly?
Come, sisters, cheer we up his sprites,
And show the best of our delights:
I'll charm the air to give a sound,
While you perform your antic round:
That this great king may kindly say,
Our duties did his welcome pay.
The rhyme emphasizes the supernatural nature of the witches and adds to the eerie atmosphere whenever they encounter Macbeth. The paired rhyme enhances the fantastical nature of all that is happening and is probably the reason why Macbeth was so easily misled. It had a hypnotic effect on the ruthless tyrant for he was overwhelmed by what the weird sisters said and he fell for their wicked charms. It is this which irrevocably led him to his doom.
In Act III, Scene 5, Hecate is a fourth witch, introduced in this scene; she is angered that she has not been consulted by the three sisters as they have "traded and trafficked" with Macbeth. Furthermore, the three witches have made mistakes. She tells them,
And, which is worse, all you have done
Hath been but for a wayward son,
Spiteful and wrathful: who, as others do,
Loves for his own ends, not for you. (3.5.10-13)
"Wayward son" is a metaphor for Macbeth, who has been resistant to the witches' guidance and influence.
So, Hecate tells the other witches that she will conjure "a dismal and a fatal end" for Macbeth. The fate that she will mix up for Macbeth, Hecate declares, will cause him to become overly confident, and
He shall spurn fate, scorn death, and bear
His hopes ’bove wisdom, grace, and fear.
And you all know security
Is mortals’ chiefest enemy. (3.5.30-33)
In this passage, "fate," "death," and "security," "wisdom," "grace," and "fear" are all personified as they are given the qualities of animate beings which can be the recipients of emotion.