The new characters we meet in Act III are the murderers employed by Macbeth and Hecate, the queen of the witches.
As Macbeth has become increasingly paranoid about the prophesies which were told to Banquo as well as himself, he resolves to have Banquo and his son, Fleance, killed in order to prevent Banquo’s family line from having claim to the throne he had taken. They are instructed by Macbeth to be thorough in their work-
leave no rubs nor botches in the work—
Fleance his son, that keeps him company,
Whose absence is no less material to me
Than is his father's, must embrace the fate
Of that dark hour.
Hecate appears in Act III scene v. She is angry that the witches have ensnared Macbeth without her involvement, and she has a plan to further confuse Macbeth and draw him further into the clutches of the forces of evil-
I'll catch it ere it come to ground.
And that distill'd by magic sleights
Shall raise such artificial sprites
As by the strength of their illusion
Shall draw him on to his confusion.
He shall spurn fate, scorn death, and bear
His hopes ’bove wisdom, grace, and fear.