For my 3 phase monitor I took the approach that it’s simply three single phase monitors in the one box. The only coupling between them is a common neutral. That’s true for the 4-wire “Y” configuration that’s common around these parts; I have trouble getting my head around some of the other 3 phase configurations in use around the world. That meant I could do all my testing and calibration in a single phase environment. When I eventually installed one in a real 3 phase environment, it worked fine.
I only have a single phase house so I simply run that single phase to each of “phases” on my 3 phase monitor (Clamp meter positioning - #26 by dBC). For kicks I set up some dashboards as if I did have three phases and see extremely good phase voltage balances as a result (all three voltmeters are aimed at exactly the same signal):
I know people claim to have had joy with 3 phase sketches that fabricate the other two voltage signals from the one that is being measured, but in my limited experience (Aus and NZ) that’s nowhere near accurate enough due to the large voltage imbalances between the LV phases. I can see the attraction of that approach however, given OEM’s absolute requirement of VTs for safety and isolation. Getting three wall-warts all plugged into outlets on three different phases and their outputs all run to the monitor can be very problematic. Here in Aus it’s not uncommon to have a second phase added for a dedicated purpose like air-conditioning, or the shed, so it can be close to impossible.
I’ve never been overly convinced by “long term agreement with my revenue meter” observations. That just means your errors are averaging out to zero, but any single observation like “how much power is X using right now?” could be way out.
All of these problems go away if you can get access to all three voltage signals in the one location and the best place to do that is near the meter and main breakers, but the safety issues there mean it’s outside the reach of the home hobbyist. You could potentially get a sparkie in to install a 3 phase outlet with associated 3 phase breaker etc, and then wire your 3 VTs to a 3 phase plug but that’s going to cost money. I know of quite a few 3 phase homes in Aus and NZ but none of them have a 3 phase outlet. At least here, 3 phase appliances are unheard of (except maybe for arc-welders). The houses have 3 phases (or sometimes just 2 of the 3 as flywire pointed out above) to spread the load.