Legio tertia Gallica ("Gallic Third Legion") was a legion of the Imperial Roman army founded around 49 BC by Gaius Julius Caesar for his civil war against The Republicans led by Pompey. The cognomen Gallica suggests that recruits were originally from Gaul. The legion was still active in Egypt in the early 4th century. The legion's symbol was a bull.
The legion took part in all Julius Caesar's campaigns against his enemies, including the battles of Pharsalus and Munda. Following Caesar's death, III Gallica was integrated in the army of Mark Antony, a member of the Second Triumvirate, for his campaigns against the Parthians. They were included in the army levied by Fulvia and Lucius Antonius (Antony's wife and brother) to oppose Octavian, but ended by surrendering in Perugia, in the winter of 41 BC.
III Gallica was used in Gnaeus Domitius Corbulo's campaign against the Parthians over the control of Armenia (58–63). Corbulo's successes triggered the emperor Nero's paranoia of persecution and eventually the general was forced to commit suicide. After this, III Gallica was transferred to the province of Moesia on the Danube.
In the Year of the Four Emperors in 69, the legion, and the rest of the Danubian army, aligned first with Otho, then with Vespasian. They were instrumental in the final defeat of Vitellius in the second Battle of Bedriacum and in the accession of the Flavians to the throne of Rome. This legion during its service in Syria had developed the custom of saluting the rising sun, and when dawn broke at Bedriacum they turned east to do so. The Vitellian forces thought that they were saluting reinforcements from the east and lost heart. In these years, one of the military tribunes of the III Gallica was Pliny the Younger.
After this civil war, the legion was again sent to Syria, where they fought against the Jewish rebellions of the 2nd century. They also took part in Lucius Verus' campaign (161–166) and in next Septimius Severus (197–198) campaign against the Parthian Empire, none with noteworthy success. During the reign of Roman Emperor Caracalla (211–217 AD), the Legion left an inscription amongst the Commemorative stelae of Nahr el-Kalb.
III Gallica played a central role in the early reign of Elagabalus. In 218, during Macrinus' reign, Julia Maesa went to Raphana, Syria, where the legion was based under the command of Publius Valerius Comazon. She largely donated to the legion, which, in turn, proclaimed emperor Julia Maesa's grandson, the fourteen-year-old Elagabalus, on the dawn of 16 May. On June 8, 218 near Antioch. Gannys, Elagabalus' tutor, defeated Macrinus and his son, with the help of the III Gallica and the other legions of the East.
In 219, the legion, exhausted by Elagabalus excesses, supported its commander, senator Verus, who proclaimed himself emperor. Elagabalus had Verus executed, and dispersed the legion. The legionaries were transferred namely to III Augusta, stationed in the Africa provinces. However, the following emperor, Alexander Severus, reconstituted the legion and redeployed them back in Syria.
III Gallica records then become obscure. Little is known about the legion's whereabouts, but, in 323, they were still in Syria.
One noteworthy member of III Gallica was the centurion Lucius Artorius Castus.