Quite a work of art. I remember when I was a little guy and Dad let me shoot his double barrel 12 gauge, I probably jerked the trigger and both barrels went off. Knocked me on my butt. Now if this was a 10 gauge what would happen if all three went off if you jerked the trigger......ouch!.
Various triple and even quad bbl shotguns, drillings, and other exotic combinations have been sold for years. They were always particularly popular in Germany it seems, since you could have rifle and shotgun all in one by having a drilling (ie rifle bbl or bbls along with shotgun bbls).