Three-inch chambers were an available Model 21 option from the beginning. No mention of 3-inch chambers on that page from Cody. Are the 3-inch chambers on the order sheet they say is available?
The Model 21 Magnum 20-gauge wasn't introduced until the January 2,1954, Winchester catalog --
Model 21 Magnum, January 2, 1954, Catalog.jpeg
and the special 20-gauge Magnum page from the big 1955 Winchester catalog.
Model 21 20-Gauge, 3-inch Magnum, 1955 Catalog.jpg
The first 20-gauge 3-inch Magnum shells were Western Super-X loaded with Lubaloy shot, 1 1/8 ounce of #6 Lubaloy or 1 3/16 ounce of #4 Lubaloy, which appeared in the December 15, 1954, Western Cartridge Co. price list.
Super-X 20-gauge, 3 inch Magnum, #6 Lubaloy.jpg
Super-X 20-gauge, 3-inch Magnum, #4 Lubaloy.jpg
In 1960, the big four all added chilled shot 1 1/4 ounce 20-gauge, 3-inch, magnum loads to their offerings --
Super-X 20-gauge, 3-inch, 1 1-4 ounce No. 7 1-2 load.jpg
SUPER-SPEED 20-gauge, 3-inch MAGNUM, #4.jpg
Remington Express 20-gauge 3-inch Magnum.jpg
HIGH VELOCITY 20-gauge, 3-inch Magnum #6.jpg