I have a very clean Model 12 Heavy Duck 3" chambered. I also have the original stock which is in great condition. Could someone help me with an estimated value?

Can't comment on value. I can comment on beauty. Most excellent furniture and metal piece.

Have you checked auctions to see If there are any for sale?

A lot depends on the condition of the original stock and the real length of pull with the modified stock shown, which looks like either a custom fit or recoil suppressor. I have never seen anything like that before.
Going farther I also have a deluxe heavy duck gun which I had fitted for choke tubes so I can still use and enjoy it. I did some "local" research and found the choke tubes to add value. Beyond that I see their value wildly different from one to another. Your's is very nice!