Page 3 is a quest item used in the Slug Menace quest. It is given to the player by Col. O'Niall in the form of three Fragments which the player must glue together using sea slug glue. The page is used in conjunction with Page 1 and Page 2 of Brother Maledict's holy book in order to determine how to make the special runes needed to open a mysterious door in the newly discovered shrine beneath Witchaven.
The page details a method of modifying rune essence into a mystical keystone. The method involved seems a little complex but well within your abilities.
Firstly, a piece of mined rune essence needs to be shaped into the required form,
Secondly, the mystical energies which empower runestones need to be channeled into the rune essence. This would need to be performed at the properly consecrated rune alter.
Once consecrated, the five stones of earth, air, fire, water and mind should be placed into the door of the mystical prison.
However, it should be noted that this method of shaping and empowering negates the runes use for the casting of spells.
This page details the channelling method as it pertains to the elements of mind.