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Since you posted this two days ago...I don't suppose you need help?
I think he/ she still might need help, abber_dabber.
The Witches play a key role in setting the tone of foreboding that runs throughout the rest of the play. They also provide the reader with a sense of Macbeth's tragic weakness- Ambition. They suggest to the reader that while he seems a noble person, he hides an evil side as well.
The witches act as a foil to the seemingly infallible atmosphere of Celebration and Victory in the initial part of the play. They also provide the vital catalyst for Macbeth's ambition to raise its ugly head. It is the witches, through their cleverly-worded prophesies in Act I that tempt Macbeth and cause him to even allow himself to seriously consider usurping the throne from his liege, King Duncan.
I hope this helps. :)
Well I hope my reply to this helped bring this question to your attention StudentLeatn because you gave a better answer than i would have :)