I'm wondering why you'd add CR 3 Dreaded Sorcerer to anything that was already a CR 15....
Ultimately, the CR of an encounter is up to the DM. While a foe might naturally be a CR 15 and have the abilities of a CR 3 Dreaded Sorcerer (or a CR 3 Champion), it's unlikely that the abilities granted by the Villain Class actually increase the challenge; most likely, the only boost is a slight improvement to attack, defense and hp. Thus, I wouldn't increase the CR by more than 1 for such a slight change.
Villain classes are, in general, well designed for their CR. However, combining them with powerful creatures becomes more art than science (like so much of DMing, really), as the abilities are designed to scale based upon class level and not total CR. However, the base ability scores can really turn a stupid brute into a well balanced major villain (ah, Troll Dreaded Sorcerer).
Of course, scaling them by total CR could also work. Dealing 14d6 with a ranged touch attack that ignores active (Dex and dodge) bonuses to defense
is worth a slight boost in CR, even if it only comes with +14 hp, +2 attack, +2 defense, and +2 or 3 to saves.
Edit: If you really want an
official answer then post this as an [RQ] thread
here.
