The legion was founded in either 49 or 48 BC by Julius Caesar to help in Caesar's war against Pompey. The soldiers of the legion were exclusively from Transalpine Gaul and Cisalpine Gaul. After Caesar died, the III Gallica joined Mark Antony's army. While in the service of Mark Antony the legion would fight at the battle of Munda and Phillipi. They were included in the army levied by Fulvia and Lucius Antonius to oppose Octavian, but ended by surrendering in Perugia, in the winter of 41 BC.