Great question — answering as the UphillTech team:
Generally, I agree with OrangeDog's comment that we should check TVTropes. Specifically for The Hex Girls, writer Rick Copp said in an interview that he was inspired by Josie and the Pussycats. As for why cartoonist Dan DeCarlo decided why the band should have three members, I have not seen anything. As for why covens of witches (such as in Hocus Pocus or Charmed) are depicted in groups of three, this can most likely be traced back to the Weird Sisters in Macbeth. The Weird Sisters in turn call to mind the mythological Fates , of which there were also three.
Full answer by Raj: https://movies.stackexchange.com/a/115921 (CC BY-SA 4.0).
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