What was the thematic significance of the three witches in Macbeth? In what other ways did they contribute to the play?
The thematic significance of the three witches is absolutely central to the play. Collectively, they represent arguably the most important theme of Macbeth—the dangers of ambition, especially if such ambition is sanctioned by supernatural forces. In unleashing their dark prophecy upon the world, the witches also introduce a number of themes that will have great resonance throughout the rest of the play—fate, violence, and death. The witches' prophecy alerts us to the fact that there is something dark and demonic about the path that Macbeth chooses to follow. Political life at the time was a bloody and dangerous business, full of plots and backstairs intrigue. But even by the standards of the time, Macbeth's rise to power and subsequent reign are exceptionally bloody—this is because he has made a pact with the forces of darkness to achieve what he wants and what the witches have prophesied.
The witches' role is important as it shows the dangers of going against the natural order of things and of messing with strange forces that you don't understand. Almost everybody at the time believed in the existence of witches and their malevolent power; King James I even wrote a book on the subject. The witches in Macbeth are a stark warning against the alliance of political power with the forces of darkness and the rejection of the Renaissance ideal of the Christian monarch that it entails.
In Macbeth, the three witches represent the elements of the supernatural that were common during Elizabethan times. Moreover, the witches introduce the concept of fate and destiny into the play. Although the witches foresee Macbeth's future, they do not tell him how his future will come into being. Thus, the tension between fate and free will is developed. Thematically, the witches stand for this tension.
The witches contribute to the play by providing elements of foreshadowing in the plot. The reader/viewer knows that certain events will come to pass and suspense is developed as the reader anticipates the unfolding of these events.