"A thesis statement is usually a single sentence somewhere in your first paragraph that presents your argument to the reader. The rest of the paper, the body of the essay, gathers and organizes evidence that will persuade the reader of the logic of your interpretation."
The three witches in Macbeth personify the forces of evil in the universe. They are power brokers, who offer opportunities to humans, like Macbeth to draw on the strength of their dark power to achieve a goal, for Macbeth, to be king.
"They are interpreted variously as custodians of evil, spinners of the future, and as something slightly more neutral, creatures with knowledge of the future but with limited powers."
They influence the dark and destructive forces of nature, when they encounter a willing human, these forces are unleashed and cause havoc and chaos for everything and everyone. It is important to highlight that Macbeth chooses, as in free will, to participate in the witches prophecy. Once they make the prediction, they own Macbeth.
The witches goal in Macbeth is to watch the humans create suffering, pain, death and turmoil. The witches are never directly involved in any of the murders or decisions that Macbeth makes, they provide information and Macbeth acts on it based on his own desires.
What the witches are really good at is stirring the dark passions in Macbeth and his wife. Human beings are a combination of both good and evil, each day we struggle with temptations, some to a stronger degree than others, and strive to live a good, successful life. For some, like Macbeth, with the influence of the witches, his darker nature, the evil side becomes empowered and stronger than his conscience, his morality and his integrity.
Macbeth surrenders his integrity, his dignity and everything that makes being human good and fulfilling to the witches prophecy so that he can possess power.
If, in your introduction to your essay, you have established the strong belief that Elizabethan era people had in the supernatural, you can utilize this acceptance of "outside forces" as a pivotal part of your essay on the witches should you wish to demonstrate how the personalities of these "evil sisters" affect Macbeth and the action of the tragedy, "Macbeth."
Then, do some research on these "three sisters," locating criticisms from enotes (see the enotes question-and-answer group for "Macbeth") and other sources. From these criticisms you should be able to extricate an idea that expresses a commonality among them regarding their influence upon both Macbeth and the plot. So, you could write something like this:
e.g. Having an intrinsic part in the motif of ambition conspiring with unholy forces, the three witches of "Macbeth" are lurking temptations of evil, weavers of the future, and agents of retribution against mortals. [3 opinions for a 5 paragraph essay]