In the first three scenes of Macbeth, the witches play a prominent role in foreshadowing the events to come. This element of supernatural influence helps set up an atmosphere of suspense and anticipation--the reader, just as Macbeth and Banquo, want to see if and how the prophecies of the witches will come true. In the first scene, the witches say that they will meet again when the war is over; in the next scene, the soldiers all return from battle, prompting the witches to actually meet again. The reader is given the sense that the witches thus speak the truth in their prophecies. When the witches meet Macbeth and Banquo on the road and tell them that Macbeth shall be Thane of both Cawdor and Glamis and the King of Scotland and that Banquo shall be father to a line of kings, both Macbeth and Banquo doubt the truth of the witches foretellings. The reader, however, who has previous experience with the witches, is more inclined to believe the prophecies; and therefore, an atmosphere of suspense and anticipation is developed.
The first three scenes work logically, the first presenting to us, the supernatural world of the witches, the second presenting to us the natural and the human--a great exposition to Macbeth's character in absence and the third showing how the natural and the supernatural worlds encounter in the form of Macbeth's temptation at the hands of the witches.
The second and the third scenes also clarify, almost in the form of a logical syllogism, the maxim established in the first --"Fair is foul foul is fair." The second scene presents to us the portrait of the fair Macbeth with anticipations of the foul as the bleeding soldier talks about the unidirectionality of sunlight and storms of trouble. Under the rubric of valour and military patriotism, what Macbeth is reported to perform is nevertheless, acts of acute violence. In the third act, when Macbeth's speech echoes the chiasmic play of foul and fair before his encounter with the witches, we understand his inclination toward the evil. The way his imagination throws up the bloody image of Duncan's murder to realize the prophecy of the witches at the end of the scene, clearly establishes the dark underside of Macbeth's apparent fairness.