Depends on the gun and the age of it. My BSE IS chambered for 3" shells, but it is a nice 20 gauge. A smith could do it easily enough, assuming AFTER he inspected it and found it to be worthy. One consideration - the barrels are like sewer pipes - thick enough, but if the action isn't up to it, you will shorten the life of it considerably (Like shooting hot 357's in a Jframe Smith).
There are a lot of 2-3/4" shells capable of most tasks - what do you want to do with the gun?