Put negative to the case. Positive to the pole by itself and jumper from there to one of the other two that are near each other. Are there any letters stamped by the poles? I wrote this wrong. Put a jumper from the lone pole to one of the others and then power the the other one.
so, I just learned there is such a thing as a 3 phase dc motor....well, I don't think it's what you want if this is indeed what you have. It requires a driver to control the voltage to the three legs. You want a brushed DC motor. Do you have any ordering info on this? Or a tag on the motor we can look at?