How can I reduce the pressure?

Yes this is a problem, its either simply too much pressure in the system or its over expanding due to blockage to expansion vessel or no air in expansion vessel. Drain some pressure off by opening a bleed screw on a radiator, catch water in a bucket until pressure drops to about 1.2 bar when system is cold, if it still goes to 3.0 when hot it has an expansion problem

RAWPLUMB
March 2010