If your 1 or 2 bbl. carburetor fell to 3.0 inches of mercury at prolonged full throttle, the carburetor size is correct - that's the vacuum their CFM is rated at.
For a 4 bbl. it's 1.5 Inches Hg. Anything below that will not meter correctly, and at zero will not draw fuel from the bowl.
Very complex carburetors such as Weber DCOE, IDF, Dell'Orto DLHA, Mikuni PHH, etc. will continue to meter below that, but IIRC they still need at least .5".
