There are actually four witches when you include Hecate, the Queen of the witches, who is in charge and demands to know why she has been left out of the communication with Macbeth.

The three other witches are described as hags, ugly, a bit deformed and not womanly but rather androgynous.  They embody the spirit or supernatural forces in the play that act on the side of evil, so cruelty would be an appropriate character trait.  They also talk about the power they possess over nature, they are connected to the destructive forces in nature.

"The Three Witches establish their malicious nature before meeting Macbeth and Banquo. The Three Witches tell Macbeth that he will be "Thane of Glamis!", "Thane of Cawdor!" and "king hereafter" or become the King of Scotland."

What they tell Macbeth is far more important than how they look, or he would not have sought them out for a second time.  Their words and prophecy are very tantalizing to Macbeth who receives their message with great anticipation.

"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." 

Although the three weird sisters are mystical beings, they are ugly in this play, nothing like fairies or nymphs who by definition would possess beauty and be part of nature as magical creatures of the forests or mountains. 

The three witches are campy in their Halloween-like appearance standing around a cauldron making up silly rhymes when their business is much more serious and sinister.