Betacom 24c boiler pressure too high?

Boiler pressure is indicating 3 Bar. I have taken out about 2+ gallons of water from a drain valve on the nearest radiator. But the pressure drops but then returns to around 3 Bar within less than a minute, after closing the drain valve each time.
Should the cold water inlet tap below the boiler be open or closed for normal operation and during the water drain?

DC
October 2010