From the get go, Parker Bros. would chamber a gun for whatever length cartridge the customer ordered. In theory if the customer requested a gun be chambered for 3-inch shells, the chamber would in fact be 2 7/8 inch.
Three-inch and even 3 1/4 inch 12-gauge shells were available from our North American ammunition manufacturers virtually from the beginning of factory loaded shot shells. They didn't offer heavier loads than one could get in the 2 3/4 inch shell, the advantage which many gun cranks believed in was more/better wadding. The heaviest 12-gauge loads offered back in the day were 3 1/2 drams of bulk smokeless powder or 28-grains of dense smokeless powder such as Infallible or Ballistite pushing 1 1/4 ounce of shot. Here is such a load in 3-inch Winchester LEADER cases marked as Fred Gilbert's Live Bird load by Von Lengerke & Antoine --
Fred Gilbert Winchester Leaders.jpg
And in a 2 7/8 inch Western Cartridge Co. RECORD case --
RECORD 12-gauge 2 7-8 inch #7 chilled.jpg
In a Remington ARROW 2 3/4 inch case with 28 grains of Ballistite --
ARROW 12-gauge 28-grains Ballistite, Remington Arms Co., Inc..jpg
These loads are said to be a bit higher pressure than today's SAAMI specs.
The 12-gauge 3-inch progressive burning powder load with 1 3/8 ounce of shot was introduced in the 1924/5 time frame by Western Cartridge Co. put up in their RECORD case.
12-gauge RECORD 3-inch Super-X two piece.jpg
The 12-gauge 3-inch Magnum with 1 5/8 ounce of shot was introduced in 1935 along with Winchester's Model 12 Heavy Duck --
RECORD Super-X 12-gauge 3-inch Magnum.jpg
LEADER SUPER-SPEED 12-gauge 3-inch 1 5-8 ounce #5.jpg
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