Here's the challenge. From what your telling me the 3 wire in question is in fact a feed to somewhere which also needs the ability to be turned on. Like a switched outlet. I have also seen numerous ceiling fans wired this way - fan always on - light switched.
Work with me here, I only have a few chips left. Lookin for the "Yo!"
Diplomacy is the art of saying hire a pro without actually saying hire a pro