The three witches in "Macbeth" represent evil and darkness. The witches demonstrate the external evil forces working against Macbeth specifically, but that allegorically may influence any person. They are also a metaphorical reflection of the darkness within Macbeth himself.
In "Macbeth," Shakespeare uses the three witches, sometimes called the Weird Sisters, to explore the philosophical question of predestination. Their ability to predict the future accurately raises questions about whether the events of Macbeth's life are predetermined or he is the master of his own fate. If the events of Macbeth's life are already written, the witches are simply informing him of his future. If Macbeth can choose his own destiny, the witches are manipulating Macbeth into a self-fulfilling prophesy. Shakespeare ultimately leaves the audience to form their own conclusion. One prediction, that Macbeth's companion Banquo’s children are to one day become kings, is unfulfilled at the end of the play.
In "Macbeth," the hatred that the three witches have toward humanity is underscored by the darkness and rain that accompanies their appearance. The line "Double, double toil and trouble" speaks of their wish to increase the trials and hardships of the human race. Shakespeare further separates the witches from humanity by diverging from the blank verse used in the rest of "Macbeth" to rhyming couplets used in the witch's speech.