The 3 witches don't show up a lot, but they are extremely important in the grand scheme of things. They are manipulative, mysterious, and ultimately, are evil. They give advice to Macbeth, and take orders from the Goddess of Magic herself, Hecate.

I don't really mind how they sound, as they have been portrayed as multiple different ages in different media, but as long as they sound malevolent and other-worldly, it should be fine.