I would be hard pressed to buy a 3" only gun for waterfowl anymore. I like options when it comes to ammo
3.5" shells are made for the 10 GA , anything else is a pretender that and they hurt my shoulder as opposed to the sweet 10 GA.
I bought an 870 super magnum turkey gun because it was camo. 3.5 inch chamber. Seemed like a good idea. Then I shot it. Holy cow it was loud. 22 inch barrel. Then I put 3.5s and shot it at ducks SIX times I could not stand the pain. That extra power and kick hurt my middle finger bad. Traded it for a bee hive.
When it's time to go to work, the 3.5" is the tool of the trade! You can use anything you want to play with, but when it comes time to stack 'em up, you need the power 'n all the shot you can get! Heck, look att the market gunning days! They didn't mess around!
I shoot 3 1/2 12 gauge at geese. T shot or BBB. 3 inch 2s at ducks. The 3 1/2 12 gauge gives me flexibility to hunt both with one gun.