I have a Russian sxs 12 gauge. The barrel markings show it to be 3". That is if the design around the front of the chamber area is an indication.
I wouldn't rely on that decoration to tell you chamber length. If you take the barrels out of the breech and look at the flats on the underside of them, you should see proof marks to indicate what it was built for. By way of example, if it has something like 12/76 or 12x76 stamped into it, it is a gun chambered for 3" shells. If it has 12/70 or 12x70 on the other hand, it is for 2 3/4".
As others have said, shells designated 3" are a nominal 3"
after firing, whether paper or plastic. That is what a gun marked 3" is chambered for (though you could of course use shorter shells).
