Only two (edit, should be three) 20-gauge HE-Grade Super-Fox production cards show 3-inch chambers. I went through a bit over the first 2000 graded 20-gauge production cards and found forty ordered for 3-inch shells and seven ordered for 2 7/8-inch shells. Could be more as the A.H. Fox Collectors Association, Inc. only has photocopies front of most production cards, so some could be marked as such on the back. The ones I found were in all barrel lengths, two 26-inch, nineteen 28-inch, nineteen 30-inch and seven 32-inch.
Remember, back in those times the long shells came with more/better wadding, not a heavier payload than was offered in the 2 3/4-inch 20-gauge shell.
20-gauge 3-inch Rem-UMC Arrow.jpg
3-inch IDEAL 20-gauge.jpg