This is not a quiz. I'm asking, I don't know the answer.

My memory is somewhat fuzzy on this but I seem to remember that Winchester came out with the 3" 12 gauge shell. It was either Fox or Winchester that came out with the first gun chambered for it.

465H&H

I dunno about the guns, but there was a 12 ga UMC 3 inch shell in their 1896 catalog. Winchester came out with a 3 inch Super-X shell in 1921 but probably made some before that.

According to a very knowledgeable shell collecting friend of mine, buying shotgun shells prior to WWI and for quite a while afterward was somewhat of a custom deal and the buyer could order just about anything that the paper tubes could be made for. Of course, they had "standard" loads available and they were the ones in the hardware stores.

While Winchester could have made up an experimental "first" heavy duck Model 12, it was generally conceded that AH Fox and Bert Becker did it -working in tandem. They developed the very first Super Fox HE chambered for the 3 in shell using the first unmarked box of 3 inch shells to leave the Winchester factory. About 1924-1925.

We Brits were making them since the end of the 19th Century. But these were the old 3" chamber guns that fire approximately 1 1/2" ounces of shot. Not the 3" Magnum that the OP possibly is referring to?