Avoid the Grimm Brothers tradition (pointed hat, big nose, warts, etc.). To Shakespeare’s audience, a witch was an old man (this would be a nice touch -- of course all the actors were men) or woman who practiced the Druid/Celtic traditional medicine, and who lived outside the (here Scottish) social structures, usually in a cave or forest. Depending on the rest of the costume design concept (16th-c. English? Or Scottish?), the costume should read as follows:
1. ragged, worn, non-fitting, but not patched; discarded by others, layered, capable of being wrapped around in bad weather;
2. earth colors, but with something to catch the audience’s eye (headscarf or sash), since their scenes are traditionally staged upstage and to one side of stage center;
3. silhouette: bent, rounded but thin, unfocused edges;
4. twigs, leaves, vines, bird's nests, etc. tangled in robe;
5. accessories (eye of newt, etc.) in a leather sack (not cloth) with strap over the shoulder; prop person should be imaginative, and as each item is named by the witches, another container, vial, etc. should be pulled from the sack.
6. cauldron should not be large metal type, but more an earthenware pot that can withstand the campfire.
7. faces/makeup should differentiate the three witches to provide three characters; they should not look alike; definitely no Halloween touches (beards are too obvious), but aged for sure, because they had to be past child-bearing years;"hag" means "she who would not be wooed";
8. finally, if possible with lighting and shadow, they should not appear too “real”; that is, Macbeth is unsure if they are really there or just a figment of his imagination, brought on by his victory and his ambition.
Hope this helps. If desired, write to wordprof for more details.