The TB3 is a 3-axis controller. Besides the pan and tilt motors already included with your TB3, it is possible to connect a third motor the TB3 AUX Port. The AUX Port can drive 4-wire (bipolor) stepper motors. We sell a few compatible motors in our store that plug right on in to a couple of common vendors in the space.
Most people who add a third axis motor are driving dollies and sliders, but you can add a motor to drive focus, zoom, and even aperture if you have the DIY spirit.