Faith,
If you have a stock 'mom and dad' car's engine, no matter what the make, the vacuum should be steady somewhere between 17 and 21 inches Hg (mercury) at idle speed (5 to 600 rpm). I prefer 19-21"Hg

A camshaft, different than stock can lower it - how much depends on the grind.

You have triple carbs - I almost can guarantee that one of them leaks vacuum somewhere, unless totally blocked off between the carb base and manifold. Possible leaks can be found at the throttle shafts, at the base and intermediate castings, cracked castings, missing vacuum takeoff point plugs or split hoses just to name a few areas. If triple carbs didn't look so great, my suggestion would be to put on a single 4bbl and manifold, but.....

The intake to head(s) gasket may also leak.

The timing could be set incorrectly.

The mixture screws in the middle carb may be set wrong.

The mixture screws in the end carbs may not be seated - they should not be open at all on a 3x2 setup.

You may have valve, lifter or rocker problems.

You may have a leaky piston ring (or several).

You could have a bad spark plug or wire

You can pick up a vacuum gauge cheap at Auto Zone (or whatever you have there on the Left Coast) - but these are the kind of things that can lower vacuum readings - and on a pre-computer engine are the cheapest and best diagnostic tools available.

But since I've said all this, how does it really run? Skip, idle too rough, poor gas mileage, shift lousy(if it's an auto - I don't recall). If none of those are a problem, you may be worrying over nits.