Energy Meter energy calculation from power
If each register is numerically in kW then add all three and integrate over one hour to get kilo watt hours.

You might find that the register is updated on a cycle of AC. If so, then average the power for each second and that figure can be used to integrate over 3600 seconds.

If the registers are in watt hours as the name implies then just add all three. 

Somewhere in the data sheet it will give you a value for each bit in watts or watt hours. You need to make conversions using this figure into proper units.