Why are there three witches?
Some Shakespeare scholars have speculated that the three witches in Macbeth are intended to represent the three Fates of ancient mythology. But the latter are goddesses with powers far greater than the three hags of Shakespeare's tale, and the connection is, at best, dim. Three is a recurrent figure in Macbeth. In Act I, scene iii, one of the weird sisters invokes magical powers, "Thrice to thrice, and thrice to mine/And thrice again, to make up nine" (I, iii, ll.35-36). Again, at the start of Act IV, the first witch projects the time of Macbeth's second encounter with the weird sisters by noting that "Thrice the brindled cat hath mew'd" (IV, i., l.1). There are several other instances in the play in which the number three resonates with the incantations of the witches, as for example, in the three murderers of Banquo. At an abstract level, Macbeth occurs in a twilight world that is neither day nor night, but something else. The recurrence of "three" may signify unresolved ambiguity.