Donnie,

I am also fascinated by the three barrel shotguns. Famars makes a quadruple shotgun in 28 and 410. With a sidelever and single trigger!
They are not very common, but every now and then one can find a few. I think Bonham's had a couple in their recent/last auction in San Francisco. I don't think they went for much...At least not the price of a new one from the mentioned makers...
Keep an eye on the internet auctions and dealers.