The appearance of Bruce Campbell as Quentin Beck [aka Mysterio] in "Spider-Man 3" is to play out something a like this reports Filmick:

"Peter Parker finds out about a Spider-Man movie being made and goes to the set to check it out. Avi Arad and Sam Raimi have cameos as the producer and director. Quentin Beck is the movie's special FX guy.

The fact that Peter and MJ are both in the public eye - though one can never reveal that he is - is milked for a little bit of fun too.

This brief cameo appearance by Beck is just about the full the breadth and depth of the "4th villain" mystery".