Lightweight 12

[RiverRoost]

What is the lightest 12 ga 3" auto a man kind find.

[huntinsomd]

Benelli Ultralight is likely one of the lightest 12ga semi's currently produced.

[Glades]

I have a Benelli M2 with a 21" barrel and compact stock that is pretty light (for a 12 ga).
Inertia guns in general, and Benelli's in particular, will be your starting point if you are looking for a light semi auto.