The witches are first introduced in Act 1, Scene 3, when they meet Macbeth and Banquo on the heath. They greet Macbeth with three prophecies: that he will become Thane of Cawdor, Thane of Glamis, and King of Scotland. Banquo is also given three prophecies, but they are not as specific. The witches' prophecies plant seeds of ambition in Macbeth's mind, and he begins to plot to fulfill them.
The witches appear again in Act 3, Scene 5, when Macbeth visits them in their cave. They show him a series of visions, which represent his future and the consequences of his actions. The witches also tell Macbeth that he will be defeated by "Birnam Wood" and "a man not born of woman." These prophecies lead Macbeth to believe that he is invincible, and he becomes even more ruthless in his pursuit of power.
The witches play a crucial role in Macbeth's downfall. They set the events in motion that lead to his destruction, and they provide him with the false sense of security that ultimately leads to his defeat. The witches are a powerful and dangerous force, and they represent the dark side of human nature.
In addition to creating a sense of mystery and suspense, the witches also serve to comment on the themes of the play. They represent the supernatural forces that can influence human behavior, and they show how easily people can be corrupted by ambition and power. The witches are a warning to the audience that unchecked ambition can lead to tragic consequences.