I'm remote from my Hub and can't see the exact rules I'm using*, but in general terms:
Create a Rule Machine rule like:
Trigger (Temperature Changed) /* where the Temperature device can be the virtual "Average Temperature" device */
send %value% to VirtualThermostat(CurrentTemp)
For a better explanation, see: Simple Virtual Thermostat Temp updater
and Average All Device Driver
Also, the forum search interface does return lots of useful hits:
Search results for 'virtual thermostat current temperature' - Hubitat