Do you want a hardcore, competitive 3-gun shotgun or a general purpose defense shotgun? They aren't the same animal. It's like people asking for a shotgun that's great for both skeet AND trap without an understanding of either sport. The hardcore 3-gun shotgun has a 22-23" barrel with an 8 round mag capacity (plus 1 in the chamber). It will also be choked somehow; typically with screw-in chokes or possibly fixed IC or Mod. Knocking down steel plates at 30 yards requires some choke when slugs are not an option.
The typical defense shotgun will have an 18-20 inch barrel with up to 6 or 7 round capacity (plus 1 in the chamber). Choke is very optional especially if you use Federal Tru-flight buckshot and normal slugs.
If you really want to play at 3-gun, then you're talking about a specialized shotgun to be competitive at the national level. For the occasional local match, pretty much anything goes unless you absolutely have to win. If you really want to be good and win, the cost of practice ammo will cost more than the shotgun.