You need to define a couple of things. Is the generator a 3 phase generator or an automotive alternator? Is the motor a brushless DC motor or an AC induction motor? Manufacturer and model numbers of each would help.
What kind of controller are you using to drive the brushless DC motor? Typical drives take the AC input from the line or generator, convert it to DC then chop up the DC to generate the equivalent of 3 phase variable frequency AC. The magic behind this is constant communication (feedback) from special sensors in the motor to let the controller know the exact shaft position of the motor. Can you describe your project?
Your original post talked about straight voltage. Do you mean Direct Current (DC). If so, you can rectify the three phases with 6 diodes set up as a three phase bridge.