Well, now we are back to the question of balancing. If the thing is even close to balanced then you don't need such huge stepper motors.
Also think about how big the battery must be to run three stepper motors at 2.5A each.
And no, the stepper driver doesn't output 3.1V. That voltage is almost meaningless when selecting steppers. It never appears in the stepper driver specifications because the driver doesn't control voltage. It controls current. You just have to give it enough input voltage to let it do its job. The output voltage is almost impossible to measure and there is no reason to measure it.