The 3" 12 gauge was well established prior to steel even being thought of as a material for shot. The 3 10 gauge also well-preceeded steel shot. The 3" 12 gauge was brought out to accommodate steel.
Stan, many waterfowl hunters did indeed go with the 10 gauge. Others preferred to stay with the 12 & for them there was the 3" offering. Also, keep in mind that 3" shells had been in existence long before the "Magnum" came into use in the 1920's. I recall Nash Buckingham speaking of his Father using a W W Greener hammer gun in the 1880's which was chambered for a 12 gauge 3
" shell. From this gun, he fired the "Massive" load of an ounce of shot.
