My water pressure on my combi-boiler was low (less than 1) , and so I let
in
some more (up to about 2.5). However, once I turned the radiators on, it
seems to have shoot up to over 3.

Generally speaking, if you live in not abnormally tall house, the pressure
should be set to 1 bar when cold. When hot, the water will expand,
increasing the pressure. This pressure rise should be limited to 0.5 bar,
although some people will design systems with up to 1 bar increase. If the
increase in pressure when hot exceeds this, then you have a knackered or
missing expansion vessel, which needs sorting.

If the gauge ever reads much above 3, then you have probably got (hopefully)
a knackered pressure gauge, or (dangerously and more likely) a broken
pressure relief valve, which is designed to open around 3 bar and chuck your
system's contents outside the house.

Christian.