Just trace the discharge pipe (that goes through wall for overflow/overpressure) back up under the boiler. You will find a brass block with (usually) a red knob. This is the pressure relief valve. Slightly turn and hold the knob anticlockwise to discharge. If the boiler is contantly at 3 bar and discharging all the time check that the filling loop has not been left open or that the shutoff valve is not letting water by.
Robin S, September 2008