The three witches in Macbeth have traditionally been interpreted to represent the presence of evil in the play. They stir up the dark passions in Macbeth enough so that he ends up becoming an instrument of evil, creating chaos in Scotland, disturbing the natural order of things. Usurping a good and rightful king, damaging the very order of nature with his act of regicide.
The witches represent the supernatural element in the play, they inspire evil in men's souls, control the elements of nature to a degree, and influence men's decisions as is evident in the way that they instruct Macbeth on how he can satisfy his lust for power. Rising up, as if it had been hiding in the depth of his soul waiting for the right inspiration, Macbeth's lust for power consumes him blocking out reason, logic, ethics and morality.
The three witches tempt Macbeth with a prophecy that includes him being crowned king, they tantalize without providing details or time lines or any information other than that he will be king. This leaves Macbeth with a lot of questions, that go unanswered.
Choosing murder as his method of fulfilling the prophecy, he becomes obsessed with keeping his power, something else that the witches never told him. His preoccupation with murder is also helped along by the instigation of Lady Macbeth, who sees nothing wrong in murdering the king to obtain power. Macbeth proceeds on a course, which is destined to see him lose his humanity, his sanity and his soul, encouraged and guided by the witches prophecy, of murder and mayhem, crowned king in the absence of the rightful heir, Malcolm.
Three witches are the desires inherent in Macbeth. The playwright shows us what is in his heart. They are like a mirror in which we see ourselves. Those desires already existed in Macbeth's heart. Now his evil mind finds courage to act accordingly. Had there been good thoughts in him, he would have not met the witches, which symbolize evil. As you sow, so you reap. In the same way, an evil mind, full of evil desires can only act thus. Here it shows how power, pelf and fame are integral part of our life. Some fears God and so they suppress them. Others act tremendously to fulfill. Macbeth was from the second category.
Thus those three witches are none other than evil desires of Macbeth, unfolded by Shakespeare in the play.