The fact that you are consistently getting 3 KW is an indication of a problem. Most likely ( hopefully ) one of the chargers is not connected.
I use ChargePoint chargers if I wish to quickly monitor output. the 6.6 on the charger is what it is CAPABLE of putting out.
Identical 6.6 chargers around Vegas will output from around 5 to just over 7. This is solo use so no accounting for 2nd cable
in use. Each charger will always output the same, only difference is location. I figure that it has to do with power to the charger ( EVSE ).
My Zeros are Old School so don't have anything other than old Level I charger built in. I do have external chargers which will give me
a total of around 5 KW into the battery if on a road trip. MY feeling about the difference is that the lower output 5 is probably around 200 volts.
the 6 is probably around 208, and the 7 is probably 240. The other option might be that they allow for adjustment on the EVSE so an owner
could lower the output for less stress on the system. Doubtful as all of the chargers at the signature station at NV Energy all output 6 KW whild
Atomic Testing Museum and Springs Preserve output over 7 KW. The onboard charger on any vehicle is not 100%, so you will achieve a lower amount
than delivered. My Bolt plugged in to a ChargePoint usually shows similar received to what is put out. My brother has a Kia Niro EV and says that
he generally gets around 6 if plugged into ChargePoint, and around 7 if using the Tesla Adaptor on a destination charger.