If I remember correctly, a 3 inch action will use a 2 3/4" only barrel (seen that one done a time or two and did it myself with a 3" action 1100) but the 3 1/2" action won't take a 3" barrel.
Going to a 3" barrel will limit you to 3" shells. You won't be able to 3 1/2's in that barrel. Even if they chamber, they lengthen by about 1/4" when the crimp opens on firing. You run the risk of some seriously high pressures that will eventually lead to a burst barrel (while you are holding it, no less).