Firstly, I'm not a CORGI, so don't take my answer as if I were.
Simply, to reduce pressure in a sealed CH system, you can vent off from a draincock that should be on your system. There might be one beneath your boiler, which would be handy for you to watch the pressure reading fall. Usually, you should find one of these at the lowest point of your system or on a rad valve. Relieve into a bucket.
The more serious point is about how the pressure got to be 3 bar. If you filled it to that level, then silly you. If it got there all by itself then it might indicate a problem with the boiler and you should definitely get the assistance or advice of a CORGI.
scarywalton, November 2007
