The witches are manipulating Macbeth in this scene, which is where he first meets them.
In the beginning of the play, the witches tell Macbeth that they have three prophecies for him. We have met these particular witches in Act 1, Scene 1 when they discussed how they would meet Macbeth later. The prophecies tell Macbeth that 1) he will be Thane of Cawdor, 2) he will be king, and 3) Banquo’s sons will be king.
The interaction between Macbeth and the witches is an interesting one. His friend Banquo, for example, is suspicious of the witches from the start. Macbeth, on the other hand, is intrigued by what they have to say because he likes what they have to say. They are pricking his ambition. Even though Banquo is also told something he would supposedly like to hear, that his sons will be king, he is not as interested.
Banquo seems disturbed and possibly frightened by the witches from the start.
What are these
So wither'd and so wild in their attire,
That look not like the inhabitants o' the earth,
And yet are on't? Live you? or are you aught
That man may question? (Act 3, Scene 1)
He also makes a comment about their “beards,” questioning whether they are women or men. Macbeth, on the other hand, just seems curious about the witches and not bothered by their appearance.
Speak, if you can: what are you? (Act 3, Scene 1)
After listening to the witches, Banquo is even less impressed. He had asked them about himself, and learned his prophecy, but didn’t like it. It doesn't seem like he is an ambitious man. He either doesn’t believe the witches or thinks they are unholy.
Macbeth asks the witches where they get this “this strange intelligence,” but they just leave. Banquo dismisses them as insignificant.
The earth hath bubbles, as the water has,
And these are of them. (Act 3, Scene 1)
Banquo asks Macbeth if they have “eaten on the insane root/That takes the reason prisoner” and then the two of them review the prophecies musingly, or perhaps jokingly before they’re interrupted. They are ready to forget the whole thing, but the witches are apparently on to something.
Banquo and Macbeth are called before King Duncan, and the most amazing thing happens. The first of the prophecy comes true, and Macbeth is made Thane of Cawdor. Banquo is surprised that “the devil” could “speak true” (Act 1, Scene 3).
Macbeth, however, is thrilled. He sees all of his dreams coming true. The witches have ignited his ambition. Suddenly he sees everything laid at his feet, and nothing can stop him. Where before he was only a loyal solider, not he is a king. The witches did not make him ambitious, but they awoke this ambition. They were counting on it.
When Macbeth finds out that Malcolm is named king, however, he gets extremely upset. There is absolutely no reason why he should be named king. The king has two sons! Macbeth is a distant relative. Yet he wants it now! The witches have planted the idea in his head with this prophecy. They are toying with him like a cat with a mouse. Now he wants everything they promised him. They promised him this, and so he wants it, whether he deserves it or not. So he will take it. They said it was his, so it is his.
From this point on, Macbeth will go on an ambition-charged murder spree to get and keep what he wants. By the end of the play he will be unrecognizable, even to himself. Shakespeare's message here with the witches is that sometimes good people, given the right circumstances, can do very bad things. Ambition can drive people to do things you never thought they would do. It does not always take supernatural intervention.