The three witches, also known as the Weird Sisters, who appear in "Macbeth" are important to the play as a catalyst that propels Macbeth to pursue his ambition to become king. The witches are also important to the play as symbols of fate, temptation, evil and the supernatural.
The three witches confront Macbeth at the beginning of the play with the prophecy that he is to one day become king. This prophecy gives Macbeth assurance that his ambitions are in line with destiny, and he feels empowered to pursue his ambition regardless of the consequences. The witches are shown to be paranormal beings distinguished by their beards, their ability to prophesy and their rhyming speech.
The witches' prediction that Macbeth is to become king can be seen as a self-fulfilling prophecy and a temptation, since Macbeth is convinced by their words to act further his ambition toward being king. However, the witches are shown to be truly prophetic when they correctly predict that Birnam Wood will come to Dunsinane.
The fact that the withces are a paranormal trio of sisters with prophetic abilities makes them comparable to the three Fates of Greek and Norse mythology, who determine the course of human lives by weaving together and then cutting "threads of life."