How to measure the total current driven in a building by observing the electricity meter?
Electricity meters show power


No, they show power integrated over a time period (energy) i.e. kWhours and not kW actually being consumed in a particular moment. On this basis, you cannot know what the load current is in a particular "instant".

You could set a stop watch and record the change in kWhours. Then convert this back to kW (knowing the time period elapsed on the stop watch). Then estimate average RMS current over that time period but you'll have to take a stab at power factor which by no means will be either unity or constant over various load conditions that the 3ph motor might encounter over that time period.