I have two two-barreled combination guns (German term "Zweirling"), one a Savage 24F in 12 Ga/.223, and a SA M6 Scout in .410/.22RF.

Neither one is exactly muzzle-light and lively for shotgunning birds, but that is to be expected.  I imagine adding a third barrel would make them less handy, as well as heavier.

These multi-barreled guns were an artifact of unique German/Euro hunting laws, which, thankfully, have never pertained much anywhere else.  Very expensive to have 'em all shoot to same point of aim, and a lot of the examples seen in the USA are war-bring-backs in wierd, Euro calibers.

After some tweaking, both of mine are useful and reliable within their limits.  Neither is as good as a comparable single shot rifle or SG would be in their respective roles.  Sometimes I wonder what I was thinking when I bought them, but they are part of the family now, and besides, they have both appreciated so much that I would not want to sell them.  Keep 'em around for trading wampum, should the time come.  Better to trade single shots than something else, if you get my drift.

Either one will accept caliber conversions from http://www.mcace.com/shotguninserts.htm.  Such inserts, carefully thought out, could be a highly useful accessory.  I urge folks to have a close look at the site linked, and to stock up while the getting is good.

Disclaimer: no commercial interest.