Power consumed by the Resistive load
Given the circuit diagram you have provided with the B(lue) phase disconnected and no neutral shown then your answer of 500 watts is perfectly correct with a line voltage of 100 V.

The written solution to this problem will only be "correct" if a neutral line was present because then you can see that each load resistor will receive a phase voltage of 100/\$\sqrt3\$ = 57.74 volts and, using voltage squared divided by resistance for power, you get 333.33 watts per resistor making 666.66 wats in total.

I will also add that the "correct" answer of 666.54 watts is naive and that the way the solution calculates current is also naive. The way the written solution rounds the correct phase voltage of 57.735027 to 57.73 is very careless.