In this scene, Macbeth returns to visit the Weird Sisters (Witches) to get more information about their original prophecy that he would become king. He wants to find out more particulars not only about what they told him in the beginning of the play, but why they told Banquo that his descendants would be kings. So the role of the witches in this scene is to advance the plot and increase the suspense.
The witches answer his questions. They conjure up horrible ghosts, each of which offers a prediction to dispel Macbeth’s fears. First, a floating head warns him to beware Macduff. Macbeth says that he has already figured that out. Then a bloody child appears and tells him "none born of woman" will harm him. Next, a child with a crown holding a tree tells him that he is safe until Birnam Wood comes to Dunsinane. Finally, a procession of eight crowned kings walks by, the last one carrying a mirror. Banquo’s ghost walks by at the end of the line. Macbeth wants to know the meaning of the final vision, but the witches perform a bizarre dance and vanish.
The witches' prophecies ease his mind, but as we find out later in the play, what Macbeth has thought to be impossible prophecies- i.e., he can only be hurt by someone that is "not born of a woman" and he will be safe until "Birnum Wood" comes to the castle, turn out to be true because he is killed by one who was born by a Caesarian section procedure and the advancing army is carrying twigs and branches and it looks, indeed, like Birnum Wood coming to get him.
Macbeth shall never be defeated, until
Great Birnam wood shall come to
High Dunsinane hill against him
Be bloody, bold, and resolute; laugh to scorn
The power of man, for no one given birth to by a woman Shall harm Macbeth
At the end of the scene, Lennox arrives and tells Macbeth that Macduff has fled to England. Macbeth sends murderers to capture Macduff’s castle and to kill Macduff’s wife and children.
The opening of the scene in question is very important because Shakespeare, for the only time in the play shows us a detailed image of the way the witches process the evil of the world. All the waste-things that go into the cauldron to produce the evil charm underline the dark underside of nature and once again imply the interpenetration of the natural and the supernatural, as always in Shakespeare.
The foremost function of the witches in this scene, as developed from the first scene and their temptation in act I scene III, is the climaxing of the art of equivocation. They are masterful speech-manipulators and purveyors of linguistic deception, falsehood and illusion. They instill in Macbeth the false confidence of an indestructible ego by giving him the impression of immortality. In all their pronouncements, there are absurd hypotheses which, once their linguistic guise is taken away, is not that absurd after all.