Is this really an Act? It seems to have been labelled as such to fit the play into the expected five act format. Did Marlowe write it? He probably recruited a comedy writer to do it. It provides some anti-Catholic humor based on what was in the English Faust Book, and serves as a relief from the intensity of the play. Along with Act 4, the humor and special effects would have received approval by audiences.
Wagner provides a quick overview of what we shall be seeing.
There is a pointed jibe at the Catholic rites as the Pope grows angry and Mephistopheles suggests that Faustus is about to be excomuunicated (note that we see another occurence of a book):
Mephistopheles: ... We shall be
with bell, book, and
Faustus: How! Bell, book, and candle; candle, book, and bell.
Forward and backward, to curse Faustus to hell ...
This is another bout of humor featuring Robin and Rafe. It is a variation of what was in the Faust Book.