Do I add the Munitorium +1 to each macrobattery in the salvo? In other words, if I fire 3 macrobatteries, is that +3?

I believe that is how it works, yes.

It's +1 to damage. That means that if a given macrobattery hits with enough DoS for multiple hits (up to Strength) then each hit will get a +1. This can be very effective for a cruiser that drops lances and goes all batteries.