For UK, the nearest correct answer is 1.
With a 3-altimeter aircraft, the two primary altimeters would be set to Standard (1013.25) when initially cleared to a FL or, if intermediate altitude passing reports have been requested, when the final passing altitude has been vacated. In my experience, the standby altimeter is best left on QNH until top of climb, when it can be set to either 1013 or, for a short sector, to the destination QNH.
This (for the UK) is reflected in the UK AIP at ENR 1.7 - ALTIMETER SETTING PROCEDURES, paras 5 (Detailed Procedures) in particular.
This subject comes up regularly - you could try reading
here and
here for a start ...
JD
