of superstition, being superstitious themselves.  Either way, they will remember what has been said because it has been repeated so many times.

The Three Witches in Shakespeare's play Macbeth serve at least two important functions. First, the witches establish a supernatural element to the play, which contributes to the theme "fair is foul, and foul is fair." Throughout the play, Shakespeare introduces various supernatural elements to set an ominous mood, reveal certain internal qualities of characters, and heighten the suspense of the drama. The Three Witches themselves introduce the idea that "fair is foul, and foul is fair," which runs throughout the entire play. As the play progresses, characters continually misinterpret behaviors, signs, and prophecies.

In addition to establishing a supernatural element and introducing a primary theme of the play, the Three Witches also serve as instruments of fate by delivering their prophecies to Macbeth and Banquo. When the Three Witches initially meet Macbeth and Banquo on the heath, they call Macbeth the Thane of Glamis, the Thane of Cawdor, and the future king of Scotland. Once Macbeth discovers that he has been given the title Thane of Cawdor, he recognizes that the witches' prophecies were true and immediately contemplates murdering King Duncan. Essentially, the witches' prophecies incite Macbeth's ambition, which motivates him to commit regicide. Later on, the Three Witches offer Macbeth several seemingly favorable prophecies, which influence him to become overconfident. Macbeth then becomes a victim of hubris when he faces Macduff in hand-to-hand combat in the final battle. Overall, the Three Witches establish a supernatural element to the play and deliver significant prophecies, which incite Macbeth's ambition.

In William Shalespeare's play Macbeth, the three witches are very important. They set the mood, set the plot, and to bring the supernatural into the play. 

First, the witches set the mood of the entire play. Given the witches are seen in the opening scene of the first act, the mood is defined as ominous and dark.

Second, the witches are important for their actual role in the play. The witches are responsible for providing Macbeth with the prophecies. Without these prophecies, Macbeth's ambition would not have ruled him (which would also have kept him from murdering Duncan and being killed himself).

Lastly, the witches add a supernatural element to the play. Shakespeare wrote Macbeth for King James I. King James was highly superstitious and enjoyed the supernatural. Therefore, the inclusion of the witches in the play offered homage to the king.

In Macbeth, the witches were very important in making his decision to take the throne of Scotland from Duncan.  Macbeth had this idea prior to meeting up with the witches but once they told their prophecy it was confirmed in his mind.  Once Macbeth tells his wife about the prophecy, she becomes a driving force in his striving for power.  Without the meeting with the witches, Macbeth probably would not have had the strength to murder Duncan.  The entire shape of the story would be different if the witches were excluded.

It is also possible that the Weird Sisters do not have the ability to prophesy the future and actually only tell Macbeth the things they do in order to manipulate him into corrupting himself with his own ambition. After all, when they say in the first scene that "Fair is foul, and foul is fair," they imply that they are going to make good things seem bad and bad things seem good, a manipulative activity. Then, when they tell him that he will be made the Thane of Cawdor, they aren't actually telling him the future; they are telling him something that has already happened (in the scene prior), though Macbeth isn't yet aware that it has happened. When Ross tells Macbeth that Duncan has named him Thane of Cawdor, it seems like the Weird Sisters predicted the future. 

To be sure, they do have some supernatural ability: they vanish, mystically appear, and even conjure some freaky apparitions (with the help of Hecate). In Shakespeare's era, people didn't like the idea that fate controlled their futures, that no matter what they did, a specific something was sure to happen; they much preferred to believe they had some agency and control in their own lives. Shakespeare certainly seems to leave the question about whether the Weird Sisters truly know the future or simply manipulate Macbeth for their own amusement open.

In the play Macbeth, the three witches (or the weird sisters) are supernatural elements who are harbingers of fate and destiny.  The witches are the first characters introduced to the audience, and in that first scene, they offer the paradox of "Fair is foul, and foul is fair" to set up the thematic foundation of the play.  They caution the audience that not all is as it seems, and as the events progress, the paradox unfolds. The witches next appear in Act 1, scene 3 when they meet Macbeth and Banquo on their way home from battle, and they give the men the prophecy regarding becoming king.  Here, the witches are only offering the truth of what is to come.  The final appearance of the witches is in Act 4, scene 1 when Macbeth seeks them out to get a prophecy to make him feel secure in his position as king.  The witches show him four apparitions, and they eventually all prove true, even though Macbeth dismisses most of them because he misinterprets their meanings.  So the witches simply relay the truth about future events.

In addition to what's been written above, the witches in Shakespeare's Macbeth contribute to the theme of equivocation present in the play.

They are the master equivocators.  They tell Macbeth predictions that seem fool proof, but in fact are not.  Who could guess that soldiers would camouflage themselves with branches and leaves so that when they move in toward Macbeth's castle they, from a distance, look like trees moving?  Who could guess that Macduff's mother died in child birth before Macduff was born, so that he actually wasn't born of woman, but born of a "body."

The Macbeths equivocate, in a sense, when they behave so hospitably toward Duncan then assassinate him.  Macbeth equivocates when he tells Banquo they will talk the next day, knowing that the murderers will kill Banquo later that day.  Malcolom suspects Macduff of equivocation when Macduff comes to England to join Malcolm in the fight against Macbeth. 

Further, this theme is emphasized by the porter in the famous comic relief scene that separates Duncan's slaying from the discovery of his body.

Again, the witches are the masters of equivocation.  Macbeth is quite an equivocator himself, but the witches are certainly one up on him.   

The three witches, or Weird Sisters, were used to appeal to those interest in the occult, which many people in court were, especially King James. So while Shakespeare used this as a way to please the royalty and people that were paying him for his works, they also function as a way to drive the plot and create tension within the viewer. They drive the plot because they foreshadow/foretell the future action of the play. When Macbeth is unhappy and fears Banquo, he goes back to them as a source of knowledge. However they also create tension because the classic question arises: Is it their fault of telling Macbeth this prophecy (which may or may not have actually come true) or is it Macbeth's own greed and ambition that destroys him? So they are essential to the ideas of fate and occult within the play.

In the play 'Macbeth' by William Shakespeare, one of the main roles of the three witches is to cast a general aura of mystique, magic and menace over the entire play right from the start. Different critics have different ideas as to why this is. Some are concerned with the alleged interset of King James in witchcraft - apparently there were things in his background which made him superstitious so you may want to check this out. Also, critics have explored the area of genderless beings - these 'creatures' were supposed to make people feel 'afeard' as one could not be sure what they were (note Lady macbeth's request to 'unsex me' too in connection with this and check out too.)

The three Weird Sisters are important in Macbeth in primarily because they provide the inciting incident: the moment when Macbeth, the protagonist, is thrust into the action, and his inner conflict begins. Prior to Macbeth meeting the three witches, his loyalties are undivided, and his character seems settled. He has fought valiantly against the enemies of the Scots crown, defeating the rebel Macdonwald and putting down the Norwegian invasion. He is praised and promised a new title by the king as a reward for his faithful service, though Macbeth does not yet know this when he encounters the witches.

However, once Macbeth hears the witches' prophecy that he will be Thane of Cawdor and also king, he immediately begins to consider what would have to happen for them to come true. When he learns that he has been made the Thane of Cawdor, as the witches said he would, he begins to yearn to be king, the other title they promised him.

When he hears King Duncan name his son, Malcolm, the heir to the throne, Macbeth realizes that he will either have to give up the dream of becoming king or commit to taking the throne through violence. He addresses the stars, saying,

Stars, hide your firesLet not light see my black and deep desires;The eye wink at the hand; yet let that beWhich the eye fears when it is done to see (1.4.50–53).

He asks, then, that the stars go dark so that there will be no light for anyone else to see what Macbeth now plans: to kill Duncan and take his crown. Macbeth also does not want his eyes to see what his hand will do when he uses that hand to kill the king, likely because he knows how terrible a crime it would be. Without the witches' interference, it seems unlikely that Macbeth’s ambition would ever have driven him to such an action.

The witches ultimately play a significant role in Macbeth's downfall. Not only do they plant the idea of being king in his mind, but they also offer him other prophecies that he incorrectly interprets as proof he cannot be defeated. He only realizes his mistake just before he is slain by Macduff. Of course, the extent to which the witches are to blame for Macbeth's downfall is a matter of great debate that cuts to the heart of Macbeth's character and agency. Some believe Macbeth to be a victim of the witches, who deliberately manipulate him into committing evil acts and ultimately lead him to his doom. It's also possible to interpret the witches as neutral actors who simply communicate what will come to pass, meaning that Macbeth bears full responsibility how he interprets and acts upon their predictions.

'Witches' is ambiguous. Confusion has largely arisen because the Folio text refers to them in stage directions and speech prefixes as 'witches'. They call themselves the 'Weird Sisters' and Banquo and Macbeth refer to them as such. The only time the word 'witch' is actually heard in the theatre is in line 6 of I,iii when the First Witch quotes the words of the sailor's wife as the supreme insult for which her husband must be tortured.

'Weird' did not come into its modern usage before the 19th Century - it meant Destiny or Fate, and foreknowledge is clearly the Sisters' chief function. But their powers remain ambiguous - they are actively malicious to the 'master o' the Tyger' but don't have the power to destroy him. They appear to Macbeth at will - theirs and his - but confine their interference to prediction. These powers - to hex and predict - were attributed to village witches, but the Weird Sisters are more decisively supernatural, and the ambiguity (of nature, and of power) is fundamental to the ambiguities of experience and knowledge (does Banquo's ghost 'exist' for any but Macbeth, for instance? And what about the Dagger?) as the play develops.