The witches really do not predict Macbeth’s future as much as make assumptions. Everyone knew of Macbeth’s ambition so anyone could guess that he would love to be in power. If you carefully examine the witches prophesy you will find vague predictions that fee off of the reactions of Macbeth as well as Banquo. Not to undercut the witches by any means because they did possess the power of persuasion, but that was all it was. They had the ability to plant the seed in the mind of their victim and let it grow, occasionally watering it from time to time.
The three witches that are introduced at the beginning of the play are responsible for the introduction of the ideas that caused Duncan’s death and Macbeth’s destruction but not for Macbeth’s actions themselves. They recount to Macbeth three prophecies; that Macbeth will be: 1) Thane of Cawdor, 2) Thane of Glamis, and 3) King. Macbeth welcomes the ideas spawned from the witches’ prophecies, which is what triggered the spiral of events in this story. Macbeth eventually followed through with killing King Duncan. It was sometimes thought that the witches had the ability to reverse the natural order of things. This brings to the play the idea of fate and the role it has in the play. One can wonder if Macbeth ever had a chance of doing what was right after he heard the witches’ prophecies. However, it is more realistic to believe that Macbeth was responsible for his own actions throughout the play and in the end it was he that made the final decisions. The witches could predict the future, they can add temptation, and influence Macbeth, but they cannot control his destiny. Macbeth creates his own misery when he is driven by the guilt of his actions. This causes him to become insecure about his actions, which causes him to commit more murders. The witches offer great enticement, but in the end, it is each individual’s decision to fall for the temptation, or to be strong enough to resist their appeal.
The witches can hail Macbeth with his future titles because they can see the future. They don't predict his future in the sense that they guess at it; their address is a statement of fact. Does this mean that Macbeth has no free will? Of course not. The witches can't control what he does--they can only report what they have already seen him do.
So how? By hailing him with his present and future titles.