Some 3' slug guns that aren't too expensive:
Remington 870 express
Winchester 1300
Browning Bps
All the ones listed above are pump guns, the 3' semi's you might be able to find at a reasonable price would be used, they could include/but not limited to Remington 1100 Magnum or Remington 1187, Browning Auto 5 Magnum to list a couple, not sure where you're located but its always best to check pawn shops and/or classifieds in local paper to help find good buys! I really wish the pawn shops in Canada could sell guns, then I'd be all set!
NS