Skitch, a three frame 12ga Parker is certainly not common. I do not believe the Parker records contain information on how many guns in a particular gauge were built on each frame size. A three frame was standard for 10ga guns but many 10ga were built on 2 frames. I believe Parker Bros. used the 3 frame for 12ga guns to meet a customers request for a specified overall gun weight which would be on the heavy side 8+ pounds. What does your gun weigh? I have a 3 frame 12ga VH and it was ordered to be 9 pounds even. I suspect your gun was ordered for "fowling" and a heavy weight requested. I suggest you get a research letter from the PGCA.