Just because the witches say something is going to happen, doesn't mean they have seen the future. It certainly seems that way, though, doesn't it? But if all of the things that befall Macbeth and Lady Macbeth are predestined, or fated to happen, then we don't have much of a tragedy do we? For a real tragedy, we need a tragic hero who is brought down by a fault in his character that leads him to make unwise and destructive choices. If everything was already decied before hand by fate, then there's no way to hold Macbeth responsible for his actions; everything was going to happen anyway.
No, there's no fate involved. What we do have, however, are three very clever, insightful witches and one very ambitious, cold-blooded woman. Together they will prove what fate really is: Fate is not some supernatural knowledge of future events; no, one's Fate is tied directly to one's character and the choices one makes based on the strengths, or, in Macbeth's case, the weaknesses of one's character.
If you know enough facts about a person and his or her character and psychological make-up, and then know the choices an individual will face, you can pretty well predict what actions the person will take. Then, putting two and two together, you can rather reliably predict where those choices will ultimately lead.
In Shakespeare's Macbeth, the witches and their prophecies reveal the theme/idea of fate. Fate, here, is related to the idea of predestination vs. free will. Do the witches simply know what Macbeth will do or do they cause him to do it? Even if they only know the future, how much free will does Macbeth really have? If they cause him to do it, then how much free will does he have? The idea of fate goes back at least as far as the Greeks. The issue of predestination vs. free will was a contemporary controversy for Shakespeare. The issue of Macbeth's responsibility as a tragic figure is made more complex by the issue of his fate and free will.