Immediate fix is to let some water out of the system either via drain cock if you have one or by opening bleed valve on a rad and catching the output in a bucket. That will reduce the pressure and do it soon.
After that check the filling loop valve first. Your system should operate at iro 1.5 bar. Mains water pressure is iro 2.5 - 3 bar. So if the valve is open, even just a bit, then the system pressure will rise. If it's static at around 3 bar that sort of says this might just be all you need to do since the pressure inside the system and mains are now at equilibrium.
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