What did the witches plot in Act 1 Scene 3?

Witches held the same importance at the time of the writing and first performing of the Shakespearean play 'Macbeth' as they did at the time of the real, historical Macbeth [c. 1014-August 15, 1057]. In the seventeenth century, as in the eleventh, witches were believed to be in league with the devil. That was why Banquo told Macbeth to distrust the reasons for the witches making themselves and their predictions known, in Act 1 Scene 3 Lines 123-126.