The three witches have been traditionally seen as being akin to the "Fates" who calculate to spin and measure the course of events in human lives. To what extent do you see the Witches as being deterministic of Macbeth's fate? Could he have avoided their ominous predicitons?
The three witches offered Macbeth the apple, Lady Macbeth coaxed him into taking a bite, and then it tasted so good, he ate the whole thing. (Oops! wrong story!) The witches provide the atmosphere and the impetus to get the plot moving, and they also add the gothic elements that represent Macbeth's evil side of ambition and love of power. I think they're so much more than just dramatic window dressing. I can't imagine the play without them! I would go so far as to give the witches enough credit to sense that Macbeth needed very little encouragement to fall prey to their predictions. Perhaps part of their powers is the ability to know what humans are capable of. Did they realize that Lady Macbeth would be the good wife and emasculate Macbeth into killing Duncan? Perhaps this is the real power of the three witches.
You could argue that the witches did not influence Macbeth to do something he would not have done already. Macbeth was pretty quick to jump at the idea of being king. When Malcolm was named Duncan's successor, it did not take him long to think that Malcolm was a step upon which he had to fall down or overleap.
The witches used the power of suggestion to manipulate Macbeth and his wife Lady Macbeth. While both characters could have made other choices, the witches probably knew that they would be terribly tempted by the allure of the throne. The answer to your question hinges on what the witches really knew about Macbeth and his wife. There is no doubt that they knew how to push the right buttons.
The witches are merely an enabler for Macbeth to pursue his dark side. They give him information and he interprets it to suit his purposes. They are not the cause of Macbeth's downfall, he is.
Maybe this should be a new post, but the "apple plot" has been a long-standing interest of mine. In what ways do you think Shakespeare's is mimicking the Garden of Eden? Are Macbeth and his wife, like Adam and Eve, cursed with knowledge? Consider the references to the serpent, and the dual nature of Macbeth and L Macbeth - neither is strong at the same time. She is strong when he is weak, and vice versa. Almost as if they are symbiotic, as if they share a component (like a rib).
bmadnick hit the story right with the apple plot. It's Lady Macbeth who seduces Macbeth into doing the deed--I'll always see Judi Densch (sp) in that role, cajoling her husband, and then kissing him passionately so that he hardly has a chance to say no. And I saw another version of the play, I cannot remember who directed or performed, where Lady Macbeth plays that scene in bed, emphasizing the sexual power she has over him. It is strange, though, a man who is first depicted as such a warrior by his fellow soldiers so easily seduced by his wife--with the intervening scene being that with the witches. I also like bmadnick's reference to Lady M emasculating her hubby, because the woman does it through sexual power, which suggests that primitive fear going way back concerning women (and suggesting why men who fear their power want to keep them veiled).
I agree that they perhaps don't have the power, but is it maybe like getting a rotten prediction in a fortune cookie? That is, if Macbeth hadn't been predisposed to think that his fate awaited him regardless, would he have been as likely to go ahead? Or do you think he would've found any excuse to fulfill his greed and ego? If the latter, why does Shakespeare put the witches in the stew (if you will!) at all? Is it merely theatrical appeal?
The three witches don't have the power to determine Macbeth's fate. All they can do is tempt Macbeth with their ambiguous predictions. Appealing to Macbeth's ambitious nature is all it takes to get Macbeth and Lady Macbeth to determine their own fates. It is Macbeth who allows his evil side to dominate him from the time he kills Duncan until he is killed.
Could it also be considered that the witches do hold some magical powers over Macbeth. The common assumption is that they simply lead him to make his own decision but we do know that they are magical, that they brew potions and have the power of predictions. Maybe in telling Macbeth what will happen they are entrancing him? It is of course easy to dismiss this argument due to his uncertainty and Lady Macbeths influence over him (as well as Banquo's ability to see through their predictions - he however does also end up dead in the play), but she is also witch like in nature. Playing with unnatural forces when asking to be "unsexed".