You’ll be fine with a 12 Gauge 3” Magnum. While I own a 3-1/2” Magnum, I rarely shoot 3-1/2” shells.
We hunt Ducks a little differently here in the States and where I am from, we wade into the flooded timber with chest waders and hide next to a tree on the shadow side. Decoys are placed in the water and we speak to the Ducks with a Duck Call to coax them in. It is not unusual to have a Duck land in the Decoys inside of 20 yards.
In many cases, 3” shells are overkill and 2-3/4” shells will do just fine.
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