Synchronous Generator Resistive Load Characteristics?
Use a triple bank of switches that simultaneously switch the three loads to the three phases. Using pots is not a good idea because as the wiper gets close to one end of the pot, the power in that small fraction of resistance is going to sky rocket and burn the pot.

Alternatively make a 3 phase rectifier (6 diodes) and have one adjustable load on the rectified output. It's not ideal because there will be pulses of current taken from the windings but it may be a useful experiment.