v3 is energy consumption which has to come from the smart meter and can only be configured on the meter push.
I don’t use the Fronius push at all given the bugs. I use my own RPi application which works 100% accurately and has for all recent Fronius firmware versions regardless of any bugs people have reported with the native push. The JSON API has always worked fine.
From GetPowerFlowRealtimeData.fcgi I grab v1=E_Total.
From GetMeterRealtimeData.cgi I grab v3=EnergyReal_WAC_Sum_Consumed and v6=Voltage_AC_Phase_1.
I push them up as a single 5 min push with c1=1. I also derive v2 and v4 (from the change in v1 and v3) because PVO has a bug that requires you to do that if you want to see standby values.