With a 3 X 1, progressive linkage means that the engine's idle, low speed cruise, and part throttle acceleration will all be on the single center carburetor. IMHO the larger "B" (#3 flange) here is far more practical than it would be in a 2 X 1, where these ranges will use both carburetors simultaneously, and split the engine vacuum between them.
The next question is: how sure are you that you have 3 identical "B"s? There are 3 different throttle disc sizes with very similar appearance (1-9/16", 1-11/16",and 1-3/4") and 5 different venturis from 1-9/32” to 1-5/8”.
I suggest the smallest size at 130 CFM each @ 1.5 Hg" if you have a choice.
