3" is not better, just longer. If 2 3/4" shells kill ducks at the ranges you intend to shoot at then they are just fine. You cannot kill a duck deader.
I have never shot 2 3/4" steel shells but will also be picking up a box for an old 1100 even though with the steel shot barrel you can shoot 3" through a non-magnum receiver.
I have had good luck with Winchester drylocs and Kent Fast Steel in 3".
For open water hunts I use 2's. For tighter sloughs or when I know smaller ducks (woodies, teal, robbers) are more likely I will drop down to 4's.
The only downsides to 2 3/4 inch shells are they are a little slower for the same payload as a 3" and they are tougher to find.