Your questions concerning the predictions made by the witches in Shakespeare's Macbeth highlight a prominent but ambiguous issue in the tragedy. Shakespeare is writing during the aftermath of the Protestant reformation, during which the issue of predestination came into focus. The issue of fate had been central to literature at least since the Greeks, and in Shakespeare's time the issue of fate took a different form, predestination.
No one can really answer your question. The issue is ambiguous and left up to interpretation. Do the witches cause events, or just know what's going to happen? Can they really cause or predict the future, or are they just extremely intelligent and manipulative? Were the fortune-telling spirits really present, or just figments of Macbeth's mind?
An individual can come to conclusions about this issue that satisfy him/her, but the odds of one being able to conclusively support any conclusion in such a way that will convince all others are probably astronomical, as they say.
I do not believe that the witches' predictions actually cause all of the events that seem to fulfill them. I think that they only cause those events that are done by Macbeth.
When the witches proclaim Macbeth as the Thane of Cawdor, they are making a prediction. However, it seems unlikely that their prediction had anything to do with making that happen. Similarly, I do not think they had anything to do with making "Great Birnam Wood" come to Dunsinane.
I do believe that their predictions make Macbeth think he can become king and act accordingly (kills Duncan, etc).
To all the good answers posted above, I'll dare to add a few lines of my own about what my reading suggests.
The witches' predictions not at all dictate the events. Remember in the first scene of the first act, the witches say: "Fair is foul, and foul is fair". This statement of the weird sisters, for the first time, hints at how much ambiguity and dubiousness they are going to create in Macbeth's mind throughout the whole play. Ambiguity, or conflict between appearance and reality, is one of the key themes in this tragedy. The witches just equivocate in an ambiguous way, implying that, lie in a way which seems apparently true. And this they do solely to win his trust and make him a devil like them.
The witches know that, Macbeth is already an ambitious man who needs a spur in order to rouse his ambition and let it develop fully to achieve his end. They shows him what he wanted to see. They tell him what he wanted to hear. Their predicted apparent truth arises his inner demon and instigates his voracity. Macbeth is thus illusioned. He is told that no human born of woman can kill him, neither he would be killed till the Birnam Wood come close to his fort. Later, when he finds that the Birnam Wood does not come nearer, rather soldiers hiding themselves under leaves come closer, and Macduff is found to be born out of surgery probably because of his mother's illness or death, he understands clearly that, he has been deceived by the witches' dubious prophecy. In act 5, scene 7, disillusioned Macbeth tells to Macdufff:
"Accursed be that tongue... / That palter with us in a double sense, / That keep the word of promise to our ear/ And break it to our hope."
So, it is clear that, the witches prophesies contributes to influence and instigate him towards evil deed, but does not 'dictate events'.
The witches predictions dictate events but only to a small degree. If you remember where the witches first greet Macbeth, their prediction that Macbeth would be king sets off events in motion that are not under the direction of the witches. Though their prophecies are binding, it was lady Macbeth's urging of Macbeth to murder the king that made the predictions come true. After all, if Macbeth was going to become king, could he have not waited? What if King Duncan was to suffer a heart attack at some later time? It is these things that make their predictions seem less problematic.
Only because of Lady Macbeth's urging, along with with Macbeth's quest for power do the predictions come true.