2018 Z-wave Power Meter for Economy 7 electricity meter

I am advised by my electrician that my property electricity supply is three phases and that I am only using one phase rated 100 amps.
Much as I try to identify a suitable z-wave power meter for my property I continue confused.
In the UK I have so far considered NorthQ NQ-9021-EU Z-Wave Data Logger for Electricity Meters, Aeotec ZW095C3A100 Home Energy Meter Gen5 – 100A with three clamps and AEOEZW095C1A100 Z Wave Home Energy Meter one clamp 100Amp by Aeon.
Nowhere have I identified any information why Aeotec one or three clamps required.
Those of you that have knowledge of using Power Meter for Economy 7 electricity, is it possible that you will help me please?

Ok, if you are using 1 phase then you only need one clamp.

three phases in use would need 3 clamps (to measure each phase).