Error 3 indicates that this is likely caused by a speed sensor problem.
You will need to check the speed sensor for proper alignment, cleanliness and function. it should be located near the front roller or mounted on the motor. It should be within a credit cards width of the magnet or toothed wheel. If properly adjusted check with an a multi-meter to see if it is opening and closing by manually spinning the front roller or motor flywheel. If it isn't replace the sensor.