There are, to my knowledge, two types of recievers made for the Model 870, standard 3" and the Super Mag 3.5" reciever. You have the former, which will handle 2.75" and 3" shells, but will not function with 3.5" shells.

You say you already shoot 3" shells all the time, so why do you doubt that you can use 3" shells for waterfoul hunting?

Mike