You cannot. writeDigitalPin() does not support tri-state pins (left, off, right); it only supports binary (off, on) .
Systems with motors often use one of several systems:
- quadrature encoding; https://cdn.sparkfun.com/datasheets/Robotics/How%20to%20use%20a%20quadrature%20encoder.pdf
- separate pins for each direction --- this is less common and leads to questions about what should happen if both pins are active at the same time
- analog signal in which negative indicates opposite direction
- two bits grouped together that are together interpreted as a binary value that indicates direction number; this leads to questions about what to do with the 4th value. Sometimes two stop states are defined and gray encoding is used: 00 -> stop; 01 -> right; 11 -> stop; 10 -> left
