In the third act, Verdi intended to promote the witches from the first act's gossipy coven of hags into sublime beings. But triviality stubbornly clings to the witches: they remain mixed creatures. Verdi so strongly emphasizes the contrivance, the artificiality of their magic show that they become charlatans; as often in Verdi (and in Shakespeare), those who manufacture spectacles tend to lose authority and authenticity. Indeed, self-consciously moral or immoral behavior often feels inauthentic, as if Verdi distrusted people who act except according to primary appetite.

The central prop of this act is the cauldron, appropriately enough, for the whole opera has been starting to feel like a stewpot of farfetched ingredients. The banquet scene in the second act didn't have its musical courses laid out according to protocol, but instead became a jumble; and a similar rhythm of spasm prevails in the third act—it is a series of disconnected marvels, in the true Romanticsupernatural fashion of, say, the Wolf's-Glen scene of Weber's Der Freischütz. It begins with choral incantation, the equivalent of Shakespeare's “Double, double, toil and trouble” as the witches toss disgusting ingredients into the cauldron (Tu, rospo venefico—You, poisonous toad). This is in E minor, but turns to a cheerful cackly E major as the witches stir and stir. Even the initial E minor section is disturbingly good humored: despite its minor key it bears some points of comparison in melodic contour and in mood to Harold Arlen's We’re off to see the wizard, the wonderful wizard of Oz (see ex. 38). Again we see that, in the absence of humanspectators, the witches tend to be simple, full of glee; it is when they have an audience that they start to rumble and thunder, fulminate with faux-sublime.

Parisian grand opera had to include a ballet, so for the 1865 version, Verdi inserted after the incantation a dance of the witches, with a pantomime for Hecate—oddly repeating the stage history of Shakespeare's play, with its early interpolations of two Hecate scenes.