To me, it's easier to shoot a semi than a pump. A good entry semi would be a Baikal MP-153. It has a 3.5" chamber, although you would probably be fine with 3" shells. I've been shooting a Super Black Eagle at ducks for the last 15 years, but I still recommend the MP-153 to people that want a shotgun. I know of 4 people who have bought one because of my recommendation. It's not too expensive either. 26" barrel is the most popular.
As far as shells go, I normally go through a case or two of Federal 3.5" blue box steel. Last year I got a reloader and have been pushing loads in excess of 1700 fps.