Yes, it is true that the Wyrd Sisters did present a supernatural element to Shakespeare's Macbeth, as Triune pointed out. They took the form of oracles, alternately encouraging Macbeth to kill King Duncan and take his throne, and telling him that he will be killed by a man not "born of a woman."(Act 4, Scene 1, line 80)
The other intersting aspect of these charcters is that Wyrd means "fate." Essentially, the three sisters of fate, as depicted in Norse mythology as well as Greek mythology desided to pay a visit to Scotland and ruin Macbeth's life. Before he met these sisters, Macbeth had been a good and loyal subject, and had never considered killing King Duncan. Once this idea was in his head, however, he killed his king. When the sisters told him to beware of Macduff, Macbeth had thieves kill Macduff's wife and son.
Macbeth's ambition coupled with his wife's scheming ultimately fated him to die the way he did. Macduff eventually returns, and kills Macbeth. If anyone is wondering how Macduff could not be "born of a woman," the explanation that Macduff gave was that he was "from his mother's womb untimely ripped." (Act 5, Scene 8, lines 15-16) Apparently according to Shakespeare's logic, having a C-section did not count as childbirth. | 
