Hey duck man, I kill plenty of ducks and geese every year with 3 inch shells. But consider when you are hunting. In our early seasons I kill all of the geese I want with 3 inch, but later in the year Ive been using a Remington sp10 3 1/2 inch 10 guage and I knock down many more birds than my hunting partners. But that gun costs more. I shoot a Rem. 1187 most of the time and it is three inch. I also had very good sucsess with a mosberg 835 pump before I bought the 10guage. The 835 is a 3 1/2 inch chamber but has a 10 guage bore barrel. It is a great gun for turkey too! You can find them for under $400. I shot lots of clays too! Moseberg also has a 935 out now, it is a 3 1/2 inch auto and I have heard good things. It only runs about $400 I believe! Good luck!
P.S. If you try shooting Remington Hevi-shot you will be amazed! Its better than lead ever was. Last year I was killing geese out of my SP_10 at 80 and 90 yards. Unbelievable!!! But you will be able to kill all the geese you want with only 3 inch. The cost is high but if you try it once you will love it garanteed. Knock em dead!