In the film, as Taran, Eilonwy, and Fflewddur go looking for the Black Cauldron, they eventually find it in the three witches' possession. The witches are quite unwilling to give the Cauldron up, however, even though Orwen develops a soft spot for Fflewddur. Once Taran reveals he intends to destroy the Cauldron, they laughingly trade the Cauldron for his magical sword, but it is only after the trade that they cacklingly reveal the Cauldron cannot be destroyed; the only way to break its power is for a living being to willingly enter it, and that being would never be able to leave it alive. As it turns out, obtaining the Cauldron was not a smart move on Taran's part at all; with the Witches, it would've been secure forever, though now that it is his possession, the guards of the Horned King immediately come and take it.
After the Horned King is defeated and Gurgi has broken the Cauldron's power by jumping into it, the Witches appear to retrieve the Cauldron. Fflewddur stops them by asking for a bargain to return the Cauldron. Orwen offers Taran his sword back, but Taran declines in order to initiate a different trade: the cauldron in return for them restoring Gurgi. They are reluctant, but once Fflewddur accuses them of having no real power, the witches fly into a fury, vanishing in a tornado with the Cauldron and leaving Gurgi's body behind. Gurgi awakens soon after.
Their role can be viewed as strictly neutral rather than protagonistic, antagonistic or even anti-heroic; they never pick a side between the mortals and the Horned King.
Originally, they were going to be joined as one in one skirt, before was changed in the final film release.
In the original books, the witches are Prydain's equivalent of the Fates, who similarly control the lives of mortals through the threads on their loom.