Three-inch shells have been around just about as long as cartridge shotguns have been around. Prior to repeaters, which needed a certain length shell to function reliably, there were a plethora of shell lengths. In my 1903 UMC catalogue there were 12-gauge paper shells 2 5/8, 2 3/4, 2 7/8, 3 and 3 1/4 inch. The 16-gauge was available 2 9/16, 2 3/4, 2 7/8, and 3 inch. The 20-gauge was offered in 2 1/2, 2 3/4, 2 7/8, and 3 inch. In those days these longer shells didn't pack a heavier payload, but more and better wadding for a better gas seal which many serious Pigeon shooters thought to be an advantage. The first time I see the longer shells packing a heavier load was around 1912 for the 3-inch 20-gauge for the famous Widgeon Duck Club Parker Bros. guns and the J. Stevens A & T Co. pump gun. These 3-inch 20-gauge shells packed 2 1/2 drams equiv and 7/8 ounce of shot as opposed to the max load of 2 1/4 drams equiv and 7/8 ounce of shot in the standard 2 1/2 inch 20-gauge shell!!! Several of the early Ansley H. Fox graded 20-gauge guns, circa 1912-13 were chambered for the 3-inch shell of that period.
We have found a goodly number of graded Ansley H. Fox doubles in all three gauges that were ordered with longer then standard chambers. However, none of these guns that have come to light were marked as to chamber length. With the A.H. Fox Gun Co. policy of holding chambers 1/8 inch shorter then the intended shell, a "factory" chamber for a 3-inch shell would actually measure 2 7/8 inch to the break for the forcing cone.
The only way to know if your gun left North 18th Street and Windrim Avenue chambered for 3-inch shells would be to gert a letter on it from the Savage historian -- For $40 Graded or $30 Sterlingworth (last prices I've seen quoted) you can get a factory letter on most any Ansley H. Fox shotgun (Philadelphia or Utica) from Mr. John T. Callahan, 53 Old Quarry Road, Westfield, MA 01085. The information exists on the factory work-order cards, probably 85+% of which still exist. Send him the complete serial number and a check, and he can do the rest. That would tell you the specifications of the gun when it left the factory.
Or, if you become a member of the A.H. Fox Collectors Association, Inc., you are entitled to one free graded gun factory production card look-up per year and additional look ups are $25.
http://www.foxcollectors.com/ah_fox/content/