Macbeth, like many of Shakespeare's works, was written, in part, to please King James (which is considered to be one reason why Macbeth is made out to be so evil). This includes adding the supernatural or fantasy elements to the story, since King James had an interest in the supernatural.
So were the witches based on any research or specific beliefs of the time or on any particular legend, or were they just generic witches created to fit the popular image of witches of the time?
The Weird Sisters came directly from Shakespeare's source material for the play, Raphael Holinshed's Chronicles of England, Scotland and Ireland.
It fortuned as Makbeth and Banquho iournied towards Fores, where the king then laie, they went sporting by the waie togither without other companie, saue onelie themselues, passing thorough the woods and fields, when suddenlie in the middest of a laund, there met them thrée women in strange and wild apparell, resembling creatures of elder world, whome when they attentiuelie beheld, woondering much at the sight, the first of them spake and said; 'All haile Makbeth, thane of Glammis' (for he had latelie entered into that dignitie and office by the death of his father Sinell.) The second of them said; 'Haile Makbeth thane of Cawder.' But the third said; 'All haile Makbeth that héerafter shalt be king of Scotland.'
I don't know anything about the source materials Holinshed used to create his work. But given their presence together — with one speaking of the past, another of the present (though at that moment MacBeth does not know he has been made Thane of Cawdor), and the third of the future — it seems likely enough that in the original of the story the three women represented Urðr, Verðandi, and Skuld, the Norns of Norse myth. Large parts of northern Scotland and northeastern England were Viking realms during the lifetime of the historical MacBeth, so it's not at all implausible that they might figure in stories deriving from that time.
The Three Witches were meant to represent our dark thoughts and temptations to evil. They constantly play on Macbeth's ambition.
Their speech is another key to who they are. They are the only characters in the Scottish Play that speak in a ridiculous rhyme. Though, it is part of the spells they cast, the fact that their speech is so different shows that the audience isn't supposed to know why they do what they do.
There is also the fact that they resemble the Fates of Norse and Greek mythology. The prophecies they give are sometimes self-fulfilling (example: Macbeth probably wouldn't have killed the king without the Fates telling him to do so).
I also think it was a point that Shakespeare wanted them to not be completely understood as it leads us to not like them. As instinct, we tend to not like or fear that which we don't understand.
