Tested engine batteries with out disconnecting cable from terminals. All three read 12.57.
You generally get a higher reading right after boosting/charging. For the most accurate reading, battery manufacturers generally recommend waiting 6 hours after a charge before checking voltage.
You have to disconnect (isolate) each battery before checking voltage to get a proper indication of that particular battery's health. With all batteries connected, you will always get the same reading from battery to battery.
Good idea to use a hydrometer also.
Pierce
Pierce has it right,
Also in our business starting batteries are most importnt, so we use a computer battery tester, that also checks condition of battery including the CCA Capacity of the battery. If a problem shows up, we have found it is best to replace it. No one wants a dead battery at 2 AM when they need it bad.
Checking a battery while on a charger, is about useless, we take the surface charge off, then do the check to see what the actual condition is on each battery.
FWIW