Oddly enough, I stumbled into this after I had played a bit of Duke Nukem (hail to the King, Baby!
) I had thought "I wonder if anyone would be crazy enough to actually build a three-barrelled assault rifle/machine gun." And as it turns out thanks to a mention from a poster on IMFDB.org, there in fact was.
Gentlemen, I present to you, the Soviet Prizor 3b and it's variant, the Soviet TKB-0591...
I'll have to look up some more info on these, but these were apparently developed around 1966 or so. The idea was to develop an assault rifle/machine gun that had a better hit-to-kill probability, by firing three rounds at a time with each pull of the trigger, or so I'm told. Yes, that's not a gimmick...it's three barrels, with three separate chambers all built into one platform with a 60 round magazine.
As far as is known, it was only experimental. Still, I can't imagine what the weight must've been, or how much of a pain in the ass maintenance would be if you had three separate barrels and chambers to clean.
Oh and apologies if the images are too large, hope it's not a problem.