A 3" barrel is a Magnum barrel. All 3" shells are Magnums. With 1100s, the 2-3/4" barrels have 2 gas ports, and the 3" barrels have one. Some magnums will function fine down to standard field loads, but in my experience most of them will need at least a heavy 1-1/4 ounce lead field load to work well, and some of them will not work with anything less than a 2-3/4" magnum load. No way to know until you try the gun/barrel combination.
The early Remington 3" Steel Shot barrels had a smaller gas port, but the shells were changing rapidly, and they quickly went to the same size gas port as the old 3" 1100 barrels.
Why does he want to get a 3" barrel to shoot 2-3/4" field loads?
I seem to remember you have a 20 gauge 1100. My LT20 gauge Magnum is a lot more finicky that the 12 gauges were with the 3" barrel. Works fine with a 2-3/4" chambered barrel though.