Only the TB3 Black supports external triggering. We built the TB3 to be flexible in how it is configured with the I/O port on the side. Most people use this for external triggering, but some have used it to power an additional motor, or relays for lights, or to launch missiles at the enemy.
TB3 Blacks that were shipped from January to May, 2013 did not have certain internal jumpers placed in the configuration we have today. If you are having trouble with External Triggering, this could be the issue.
Below you will see a pictures of how the jumpers were set for the early models. In that picture if you move the two lower right jumpers to be in the red boxes, you will have your TB3's i/0 port configured correctly to accept external triggers.