As is well-known by Christians, the number 3 is a spiritual number. It represents the Trinity, the completeness of all Persons of God--Creator, Redeemer, Sustainer, and, thus, completeness. In a perversion of the Chain of Being, which enters greatly into Shakespeare's play, the darkness of the preternatural world
Notwithstanding this reference to the Deity of the Elizabethans, 3 has both positive and negative connotations. While the "third time is the charm," it is often the third "strike and you're out" with other situations. For example, bad luck is said to come in threes, and, of course, this is exactly what happens for Macbeth as the third prediction that he will not be slain by anyone who has been born of woman leads him to deceive himself in a conviction that he cannot be harmed. In fact, he is more disturbed in the final scenes by the Birnam Wood moving as predicted by the witches. For, he says in Scene 8,
What's he/That was not born of woman? Such a one/Am I to fear, or none.
The witches repeat things three times to bring attention to it, and to make their information seem supernatural and otherworldly.
Repetition, the act of repeating information, is often used to bring attention to important information. The witches use the rule of three by repeating things three times. The rule of three refers to the idea that there is magic in things happening three times. So there are lots of threes. There are three witches, and they meet three times, and they talk about three a lot, or thrice, and they repeat things three times! Here’s an example:
The weird sisters, hand in hand,
Posters of the sea and land,
Thus do go about, about:(35)
Thrice to thine, and thrice to mine,
And thrice again, to make up nine.
Peace! The charm's wound up. (Act 1, Scene 3)
By repeating things three times, meeting three times, having three witches, making three prophecies, and so on, Shakespeare reinforces the rule of three. He uses the repetition and the audiences association of three with power and magic to his advantage. He knows that the audience will understand the meaning of three and appreciate it. Just by saying something three times, or saying something happened three times, he has cemented it in the audience’s mind. We know that this is important. This is why there are THREE prophecies.
All hail, Macbeth! hail to thee, Thane of Glamis!
All hail, Macbeth! hail to thee, Thane of
All hail, Macbeth, that shalt be King hereafter! (Act 1, Scene 3)
The first witch, second witch, and THIRD witch all share THREE prophecies. Of course, the THIRD witch shares the most important one, that Macbeth will be king.
Macbeth, being the superstitious man that he will be, takes the three witches’ prophecies seriously and so does his wife. Shakespeare’s audience will enjoy it all in good fun, but of course they will also appreciate the aspects of superstition, being superstitious themselves. Either way, they will remember what has been said because it has been repeated so many times.