A three phase electricity supply is where you have 3 live (positive) feeds to your property and will therefore you will need 3 sensors for your energy monitor. 3 phase systems are usually found in commercial properties and sometimes in larger homes or homes with higher electricity demands.
A three phase supply meter will usually have 8 cables connected to the meter. These will be 3 live and 1 neutral entering the meter from the grid and 3 live and 1 neutral exiting the meter to supply the property. When monitoring the supply it does not matter if you connect the sensors to live cables entering the meter or to the live cables exiting the meter