I know nothing of this machine or the controllers but it may be woth looking at the signals from the steer wheel. You have probablly already checked but normally a 3 wheel machine with two motors will have some sort of device (pot or switch card) for cutting and reversing the inside wheel when steering. If this is sending the wrong signal to your controllers you may only get drive on one wheel. There is also a chance that the controllers are not programmed correctly for 3 wheel two motor operation.