RE: 3 or 3 1/2???
If you do your part 1. have a good gunmount and a gun that fits and do alot of clays shooting in the off season 2. use a good quality decoys 3. the knowledge of where and how to place decoys 4. if you done this that means your shots will be 40 yards and under. 3" or 2 3/4 are fine. Spend some time on the patterning board to see what load gives you the best pattern.