The witches greet Macbeth with three salutations: past, present, and future.
All hail Macbeth! Hail to thee, Thane of Glamis!
We know that Macbeth's title of Thane of Glamis is of the past, since Duncan has in the previous scene announced Macbeth's promotion to Thane of Cawdor.
All Hail, Macbeth! Hail to thee, Thane of Cawdor!
This is Macbeth's present title, although he does not yet know it.
All hail, Macbeth! that shalt be King hereafter!
This last greeting is the only one that is a prophecy. Notice that the witches do not specify how that prophecy will come true. And, it is not until Macbeth receives the news that he is now the Thane of Cawdor that the idea of assassinating Duncan occurs to him.
Are the witches responsible for implanting this idea into Macbeth's mind? It does not seem likely. There are other ways that Macbeth could become the next king of Scotland.
Other factors must be considered in his decision to assassinate the king. The fact that Macbeth has been ruthlessly killing the enemy on the battlefield to meet his objective may be responsible for murderous thoughts occurring to him now. Also, Duncan's decision to stay at Macbeth's castle provides the opportunity and the urgency. The Macbeths know that must strike while Duncan is there or they may never get another opportunity.
So, what is the witches' effect on Macbeth? They seem to know the future. They seem to be evil. They seem to provide Macbeth and Lady Macbeth with an excuse to kill Duncan. But beyond that, Macbeth seems responsible for his own actions.
In Shakespeare's Macbeth, the meeting between Macbeth and the three witches serves as the catalyst for the drama. The witches introduce the idea of his being king to Macbeth, or at least make the idea concrete for him. They put fate or destiny on his side.
In other words, it is possible that Macbeth had entertained thoughts of being king before he meets the witches. It may even be likely, though there is no concrete or absolute evidence of this. But the witches give him corroboration for his thoughts, if he was having them, and, more importantly, tell him that it is his destiny to be king. That's all Macbeth, and his wife, for that matter, need to begin devising a plan and putting it into action. And this plan, of course, involves killing the present king.
The meeting, in foul weather and filled with mysteries (the weird sisters seem to vanish, for example), also furthers the themes of appearance and reality and fair and foul. Throughout the play, people and circumstances are often not as they seem. The theme of the supernatural is also furthered.
The problem Macbeth has throughout the play with Banquo is developed here, too--since Banquo is present, he knows the predictions made by the witches, and will therefore naturally suspect Macbeth of treachery when Duncan is killed. And Macbeth knows it. This makes Banquo a threat to him, which will lead Macbeth to order his killing, which leads more and more people to suspect Macbeth.
The first meeting of Macbeth and the three witches serve as an important element for the play and it is from there that Macbeth's decline starts.In fact the third prophecy acts as an instigator for Macbeth and that leads to his downfall.The super hero Macbeth turns into a weakling only after this.It is here that the seeds of overambition gets hold of Macbeth.Had it not been for this meeting Macbeth itself wont be there.