Drilling means three barrel gun, so you are making a drilling. Drillings usually have two side by side shotgun barrels, with rifle barrel beneath (such drillings were carried by WWII German pilots in Africa, for survival in case of crash), but we now see more modern drillings with rifle barrel above two shotgun barrels. Some drillings are double rifles, normal side by side double rifle arrangement, with a third barrel being shotgun, usually beneath the double rifle. These are called "double rifle drillings." Some have three rifle barrrels; no shotgun barrels. The Germans and Austrians developed this sort of thing, in a lot of different configurations, and their workmanship on old examples is simply outstanding. A lot of Europeans today hunt with drillings.