Act I, Scene 1 is essentially significant in that it establishes a mood for the play as well as creating questions in the minds of the audience about the role of the supernatural in Macbeth. The witches meet to announce that they will meet again after a battle which is not named. They announce that "Fair is foul, and foul is fair," suggesting that all in Macbeth may not be as it seems.
In Act I, Scene 3, Macbeth and Banquo meet the witches. Macbeth describes them as "wither'd and wild in their attire," and observes that they have beards. For their part, the witches greet Macbeth as Thane of Glamis and Cawdor and as "Macbeth, who shall be King hereafter!" They hail Banquo as one who "shall get kings, though thou be none," implying that his offspring will rise to the throne of Scotland.
So in Act I, Scene 3, the witches offer the prophecies that will set Macbeth on a murderous path, and Banquo toward his death. Macbeth is visibly shaken by their predictions, and when he learns that he has been given the titles of thane of Cawdor, he realizes that perhaps the crown might be within his grasp:
If chance will have me king, why, chance
may crown me
Without my stir.
It turns out that neither Macbeth nor his ambitious wife are willing to wait for "chance to have him king."