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.