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.