As far as I am aware there has never been an official sequel to the first two
Demons films.
There was Lamberto Bava's film
Demons 3: The Ogre though, which was often marketed purely as
Demons 3. Confusingly, it was also marketed as a sequel to
Ghosthouse.
Arrow Video released a more 'official'
Demons 3 in the guise of a
comic in their Limited Edition Blu-ray releases of
Demons &
Demons 2 back in 2012 too, which also deals with the issue of the 'phantom' third installment.
Welcome to the forums by the way, Zach.