I am old enough to remember when Stevens began chambering their 311's in 20 ga for 3" shells. Don't recall the exact year but was in the 1960's. I do not think SAAMI pressure specs have changed since then. The high velocity shells maintain a normal pressure level through use of lighter shot loads & slower powders.
From what I have seen on the 311's it appears all gauges are built on essentially the same frame, varying only the height & width of the standing breech & firing pin spacings. Since these guns in 12ga routinely handle the high velocity 1¼oz & magnum 1½oz loads in their 2 3/4" chambers I would not expect any problem in using 3" shells in the 20.
Your only concern will not be one of "Safety" but simply if the steel will peen out the chokes. Over the long years of steel usage this has I believe been mostly taken care of by development of better wadding, than was used in the early days. Any availab;e loads whose shot more closely approaches the density of lead will of course be more effective than steel.
"Don't" try Sky-Scrapping with steel, regardless of gauge.
