There’s a discussion of that here.
There are two approaches to three phase with IoTaWatt.
Derived reference uses one VT and derives a phase and voltage from phase shifting.
Direct reference - uses three CTs (for three phase).
Direct reference can be used for split phase US systems as well, but understand that US split phase systems at 180deg phase difference aren’t subject to variation in relative phase angle as in three phase systems. So It’s just a matter of voltage. There is a discussion in the Learn section that explains how the neutral line can drift around with varying loads and change the relative angle between phases in three phase systems. The phasor graphic is pretty easy to understand.