It is, I think, the single biggest problem of the play. Are the witches having any influence over Macbeth's actions: or are they simply foreshadowing things that he would do anyway? And as ever, with Shakespeare, there's no right answer: it depends how you read the text.
We know that the witches cast some sort of spell, shortly before the first entrance of Macbeth and Banquo:
Peace! The charm's wound up.
However, precisely what this spell does or is designed to do is never made clear in the text of the play. Moreover, they do prophecy that Macbeth will become king - but Macbeth only becomes king as a result of his own actions (killing the reigning monarch, Duncan). Would it have happened anyway, even if Macbeth had sat back and done nothing? Who knows.
The prophecies the witches make in the apparition scene also all come true: but none of them lead Macbeth to do anything that he wouldn't have been very likely to do otherwise - with the possible exception of killing Macduff's children and family ("Beware Macduff").
In short then, my reading tends to be that the witches, to paraphrase something Macbeth himself says, marshall him the way that he was already going. But then, of course, why put them in the play in the first place? If you think they do influence Macbeth - then is it really a tragedy, when the protagonist has no involvement?
As you can see, Shakespeare gives us more questions than answers - as usual!
As a typical English teacher's response, I would say both yes and no!
The prime movers of the plot in MacBeth may be the witches. They could be viewed as the cause sine qua non of the tragedy: had MacBeth not met them, he would not have gone on to murder the king. Your response will vary according to your perosnal point of view. If the witches are merely relating a prophecy, then the murder was pre-destined and the meeting with the witches immaterial and he would have become king in due course as he became Thane of Cawdor.
On a psychological level, meeting the witches may have created a self-fulfilling prophecy in MacBeth's mind. Having told him that he will be King, he feels the need to live up to that expectation. In this interpretation, the meeting with the witches is the catalyst for everything that follows.
It is important to bear in mind, however, that the witches are not the only catalyst: Lady MacBeth is equally convincing or domineering. It is possible that MacBeth may not have been tempted by the witches' prophecy had he not been receiving similar encouragement from his wife. In some ways, the witches add nothing new to the play but add an additional validation to Lady MacBeth's previous machinations.