The Benelli Pacific Flyway shotgun 3 1\2 inch magnum instruction manual states that "the ammunition magazine, depending on the type, can contain two 3 1\2" rounds or three 3" or 2 3\4" rounds. Counting a chambered round, this shotgun's ammunition capacity is 3 or 4 rounds."
Remington 870 12 gauge 3 1/2" shotguns also come with a plug that allows three 2 3/4" shells, at least according to two sources I spoke with that own one.
