The distribution on a 3 × 1 like the Offy is largely based on which linkage type is used: progressive (center carburetor is the primary) or simultaneous (idles on all 3).
Just based on visual, if it's progressive I would guess the center (#3-4) cylinder pair will run rich based on proximity to the center carburetor. Various manufacturers have modified their manifolds over the years, or published tips on curing distribution problems (Chrysler), but they all involve minor internal surgery such as ditches or obstructions ("popsicle sticks") across a port runner entrance.
Needless to say, the mixture should be better with all 3 idling, but the response will be much worse since (as said) the vacuum signal will be split between the 3.
