HECATE: Into the vessel;And fetch three ounces of the red-hair'd girlI kill'd last midnight.
I haven't read the full play (I'll have to add that to my to do list), but my immediate thought is that it's three ounces of fat; and that it's in reference to the idea that the fat of redheads was used in the making of poison. [We've noted some of these strange notions before on this blog. It was said that the blood from a red-haired man was needed to turn copper into gold, and that the urine from red-haired boys was utilised in the making of both swords and stained glass windows o_O ].
Returning to the above referenced article though I'll quote the following passage.
"[A]ccoring to Elizabethan theater scholars, actors playing Jewish roles wore red wigs as a visual shorthand of Jewry and its supposed cultural and religious associations with Satanic practice. Red hair is, therefore, an appropriate ingredient in a witches' cauldron. Moreover, seeing red hair as essentially "Jewish" would fit in well with the accepted reading that red hair was associated with lechery, one of the Seven Deadly Sins associated with Jews."
These overlapping themes are always odd and interesting. Hopefully we'll find more as we dig further.