During the Clone Wars, the Third Legion served as a legion within the Grand Army of the Republic. This unit participated in the large-scale civil war that occurred on the planet Balith in 21 BBY. Following the completion of a previous assignment, Anakin Skywalker, a Jedi General who also held the position of commanding officer for the 501st Legion, was ordered by Obi-Wan Kenobi, a High Jedi General, and the Jedi High Council to immediately assume command of the Third Legion.
The Grand Army of the Republic had a clone trooper legion known as the Third Legion, which saw action in the Clone Wars by the year 21 BBY. In that year, the legion was involved in Balith's Civil War, a major conflict that took place on the planet Balith. Jedi General Anakin Skywalker, who was the commanding officer of the 501st Legion, concluded a mission and was then instructed by Grand Master Yoda and High Jedi General Obi-Wan Kenobi during a meeting of the Jedi High Council to immediately take control of the Third Legion, due to their request for support. Skywalker's Padawan learner, Ahsoka Tano, received orders to remain at the Jedi Grand Temple located on the Republic's capital, the planet Coruscant, to complete her assigned reports and studies.
Tano went with Skywalker to a Nu-class attack/transport shuttle, where Clone Captain Rex, Skywalker's second-in-command, and another trooper were awaiting their arrival. Tano tried to convince her master to allow her to join him on his mission, but Skywalker declined and boarded the shuttle that was parked on a Temple landing platform. Soon after, subsequent to Tano's success in preventing an assassination attempt on Republic Senator Padmé Amidala by bounty hunter Aurra Sing at the Alderaan Refugee Conference, Skywalker had already returned from Balith and waited for his Padawan at the Republic Judiciary Central Detention Center on Coruscant to take custody of the arrested Sing.
The animated television series Star Wars: The Clone Wars first mentioned the Third Legion in the episode "Assassin," which was the seventh episode of its third season. The episode was released on October 22, 2010.