In the opening of the play, it is not clear that the three witches are behaving entirely malevolently in their prophecies to Macbeth and Banquo. However, their statement that "fair is foul and foul is fair" alerts us as an audience that their words are likely to be deceptive.
Later in the play, Hecate gets angry at the other witches for interacting with Macbeth without her permission. Under her guidance, their intent becomes entirely deceptive. Hecate wants them to lead Macbeth to destruction because she believes that men like Macbeth do nothing to help witches.
Therefore, she sets up three misleading prophecies for the other witches to deliver, which will give Macbeth false hope. The first is to "Beware Macduff." This leads Macbeth to kill Macduff's family, which offers him no protection and only steels Macduff against him.
The second is that "none of woman born / Shall harm Macbeth." Macduff, born of a Cesarean section, is technically not "of woman born," so he can kill Macbeth.
In the third...
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