thoracic spinal nerve 3 thoracic spinal nerve 3 the thoracic spinal nerve 3 (t3) is a spinal nerve of the thoracic segment. thoracic spinal nervethe spinal cord with spinal nerves.detailsidentifierslatinnervi spinalisfma6171anatomical terminology it originates from the spinal column from below the thoracic vertebra 3 (t3).