The witches in Macbeth are given decidedly masculine features. As Banquo tells them,
You should be women,
And yet your beards forbid me to interpret
That you are so.
By adopting male power (and even appearance), the witches are able to escape their traditional female roles while still retaining an essential femininity. Shakespeare examines the duality of gender roles in his description of the weird sisters, altering the nature of femininity itself by adding traits to the witches that have traditionally been viewed as fully masculine, especially in his time. By making the witches' gender ambiguous, Shakespeare effectively disrupts traditional gender roles.
The witches' physical appearance and power to manipulate Macbeth show us that they possess traditionally "masculine" traits. The trio is perceived as violating nature, and despite their designation as sisters, the gender of these characters is also ambiguous. These powerful female forces influence, and at times control, Macbeth's actions.
The witches embody both masculine and feminine traits, not only in their appearance but in their actions as well. They are a clear authority figure in Macbeth's life and warn him about his future, but they do so in a way that causes him to think he will never be harmed and that all of his goals will be achieved. In this way, the witches dominate and control Macbeth, embodying more traditionally masculine than feminine traits. Shakespeare heightens the witches' "unnatural" blend of masculinity and femininity to highlight the fact that the witches' control of Macbeth is itself unnatural.