First what chamber length is marked on the shotgun?
If it doesn’t say 3” you will risk wrecking the shotgun if you use 3” shells.
What is magnum shot? There’s no such thing. Steel shot is the big question mark, it depends on what kind of choke the shotgun has.
So why don’t you post some pictures of the shotguns markings and some of the muzzle details. Then you might get some assistance.
Browning Superposed shotguns are not recommended to shoot steel shot ever !!!
There are 3 inch chambered Superposed shotguns from the days of lead shot.
Bismuth shot would be suitable for Waterfowl in a Superposed shotgun.
As saskbooknut stated, if it is a true vintage Superposed, they had thin barrels and forcing cones not suitable for steel .
if you want to shoot waterfowl with it, the new nontoxic Bismuth, etc is the way to go but make sure it is not Hevi Shot!
Cat
__________________
Anytime I figure I've got this long range thing figured out, I just strap into the sling and irons and remind myself that I don't!