I know they used to make 3 barrel carbs back in the late 60's, think it was AFB. But those were monsters, 2 good size primaries and 1 LARGE secondary. That's the only 3 barrel I've ever heard of but then again I don't mess with foreign cars much.
The Carter AFB and AVS are 4bbl square bore carburetors.
It'd probably take eight of those things to run the 460.
According to what I saw, the 1.8L was able to pass emissions testing without being factory equipped with catalytic converters! The auxiliary venturi ignited a fuel mixture in the pre-combustion chamber to begin the burning process, while the main mixture is drawn through the primaries and secondaries. The auxiliary venturi and the pre-combustion chamber lengthen the burn time of the fuel mixtures to ensure a more complete burn and as a result, the nitrous oxides and the unburnt fuel is burnt as well as an extremely efficient oxidation process for CO.
Now it makes me wonder why we weren't doing this in the US?
The Honda CVCC engine used the third barrel to feed a prechamber with a nice easily ignited mixture, the flame from this would ignite the lean mixture in the main combustion chamber. These engines were prone to valve stem seal issues, causing the very tiny prechamber inlet valve to carbon up. When this happened the engine wouldn't run very well except at full throttle. The first one of these I saw, the dealer had sent it to my shop. The original Prelude used a sidedraft 3 carb assembly that Honda serviced as one.
The Holley 3 barrel was an upgrade from their 850cfm 4 barrel, it went out of favor when the 4500 Dominator came out.
I have seen a number of other 3 barrel carbs, Solex built several, one for Porsche 911a, two different styles for SAAB, the 850 Monte Carlo used a downdraft and the Sonnet used a side draft. Weber built a 3 barrel for the Porsche 911s, a 48IDA3P, and I think I have seen similar ones used as a foursome on some Italian V12s.