Why are the Three Witches referred to as the Weird Sisters?
In the Folio edition the spelling is weyward. Our modern-day meaning of weird, i.e., odd or strange, is not really accurate. Weird here comes from the Anglo-Saxon wyrd, and means fate or destiny. Thus the Weird Sisters are foretellers of Macbeth's fate.
In Shakespeare's primary source for Macbeth, Holinshed's Chronicles, the Weird Sisters are "goddesses of destinee", but they are far more sinister in Shakespeare's version. I discuss Shakespeare's changes in An Analysis of Shakespeare's Sources for Macbeth:
Notable changes are also made by Shakespeare in his depiction of Holinshed’s three Weird Sisters, and it is apparent that the alterations are implemented partially to instill trepidation in the audience. Holinshed’s sisters are ‘creatures of the elderwood . . . nymphs or fairies’ (Chronicles 268). Nymphs are generally regarded as goddesses of the mountains, forests, or waters, and they possess a great deal of youthful beauty. And similarly, fairies are defined as enchantresses, commonly taking a small and dainty human form. Holinshed’s illustration of the creatures Macbeth chances upon is far removed from the portrayal Shakespeare gives us through Banquo. Shakespeare transforms the Weird Sisters into ugly, androgynous hags, and they distinctly take on a more sinister role than was assigned to them in Holinshed’s Chronicles...(from Sources for Macbeth, Shakespeare Online).