Macbeth
Before reading the play my instinct was to say that the three witches symbolize the three fates. The number is the same and the three witches finish each other's sentences in the way that the fates are usually portrayed as doing. The fact that what the witches predict comes true, and comes true only because Macbeth acted on their prophecy (rather like how Trelawney's prophecy in Harry Potter came true only because Voldemort acted on it).The biggest difference between the witches and the fates is that (in spite of how popular culture portrays them) in their original mythology the fates do not try to cause harm. They simply do their job creating people's destiny, and occasionally recite a prophecy, without any malicious intent. The witches on the other hand are deliberately trying to lead Macbeth to corrupt his soul. The way that they hint to him that he has good things coming, just enough to make him act to gain those things, even at the expense of others. Even at the expense of his own soul. Because of this I think that the Weird Sisters represent demons, and Hecate, who reprimands them not for the harm that they have done, but for not letting her in on their fun; 'How did you dare/To trade and traffic with Macbeth/In riddles and affairs of death;/And I, the mistress of your charms,/The close contriver of all harms,/Was never call'd to bear my part,/ Or show the glory of our art?'
It appears to me that the Weird Sisters may represent demons, with Hecate representing Satan. Another possibility could be that the witches represent the potential for evil in Macbeth, easily egged on by Lady Macbeth because it is already within his capacity to commit.
The witches apply to the themes of violence and fate. In violence as they spur Macbeth onto violence in his second meeting with them, summoning visions of bleeding heads and murdered babies. And fate as they cause Macbeth, Lady Macbeth and Banquo to question whether the things they predicted would come to pass naturally, or if they will have to act to gain the prophecies.
Without the Weird Sisters the play would not have happened, unless something else took their place. They are responsible for Macbeth and Lady Macbeth resorting to violence, and all the chaos that ensues. They could have been replaced by Macbeth making a conscious decision to kill King Duncan to gain power, but that wouldn't have been as compelling.
Lady Macbeth pushed Macbeth to kill the king trusting on the words the witches enough to believe that Macbeth would become king, but not trusting enough to wait and see if he would become king without them taking action. Ultimately neither husband nor wife could live with the guilt.
(This review was originally a discussion post I wrote for an online Shakespeare class.)
