Does the power factor of a balanced 3 phase system is 1?
You can imbalance a 3 phase system with purely resistive loads. Imagine taking 10kW on one phase and 20kW on the other two. This is called imbalanced because the phase currents don't have equal magnitude. Yet, the power factor of each phase is unity.

A balanced system (balanced reactive loads) will not have a power factor of unity.