Practical shotgun shooting is all about the reload. Pick a method and practice it.
The Benelli Nova is the most popular pump in 3 gun (Heavy Metal) and action shotgun, for one reason. Its easy to load fast. They point like a shovel, IMO, but you can shove shells in them very fast, and they are reliable.
Remintron 870's and Winchester 1300's are popular as well.
Mossbergs are almost non existant. Contrary to all the claims, they dont seem to hold up well to the hard use that a competition gun sees.
Most 3 gunners shoot semi autos. Benelli M2's dominate the game by a large margin. The Remington Versamax, FN SLP, and Winchester SX3 are probably all in a tie for a very distant second place.