In addition to what's been written above, the witches in Shakespeare's Macbeth contribute to the theme of equivocation present in the play.
They are the master equivocators. They tell Macbeth predictions that seem fool proof, but in fact are not. Who could guess that soldiers would camouflage themselves with branches and leaves so that when they move in toward Macbeth's castle they, from a distance, look like trees moving? Who could guess that Macduff's mother died in child birth before Macduff was born, so that he actually wasn't born of woman, but born of a "body."
The Macbeths equivocate, in a sense, when they behave so hospitably toward Duncan then assassinate him. Macbeth equivocates when he tells Banquo they will talk the next day, knowing that the murderers will kill Banquo later that day. Malcolom suspects Macduff of equivocation when Macduff comes to England to join Malcolm in the fight against Macbeth.
Further, this theme is emphasized by the porter in the famous comic relief scene that separates Duncan's slaying from the discovery of his body.
Again, the witches are the masters of equivocation. Macbeth is quite an equivocator himself, but the witches are certainly one up on him.