Short answers, pulled from the story.
The Sanctuary of the Three Gauls was built at Lugdunum, modern Lyon, because the city sat at the junction of three newly established Imperial provinces: Gallia Aquitania, Gallia Belgica, and Gallia Lugdunensis. Rome chose the site to serve as a centralized base for Imperial governance and to federalize the unstable western provinces under the Principate.
Drusus, stepson to Augustus and provincial governor of Gaul, founded the sanctuary in rapid response to a rebellion provoked by the census of Gallia Comata in 12 BC. The inaugural day was the 1st of August, in either 12 BC or 10 BC.
The first sacerdos, or high priest, of the sanctuary was Caius Julius Vercondaridubnus, a member of the Aeduan elite who held both Roman citizenship and Gallic origins. His election reflected the Aedui tribe's longstanding status as allies, or fratres, of Rome.
The Lyon Tablet is a bronze plaque measuring 2.5 by 1.93 meters, engraved with a speech by the emperor Claudius from 48 AD arguing that Gallic chieftains should be eligible for Roman magistracies and Senate membership. It was found by a draper in 1528 in his vineyard on the site of the sanctuary and is now held in the Gallo-Roman Museum of Lyon.
The altar stood on a 50-meter marble base and was flanked by two gilt bronze winged victories, each holding a palm and a gold crown, mounted on Ionic capitals atop columns. The geographer Strabo described it as bearing inscriptions of sixty tribe names and images representing each tribe, alongside a second, larger altar.
Shortly after 198-9 AD, following his victory over Clodius Albinus at Lugdunum, Septimius Severus re-founded the Imperial cult centre and removed the image of dea Roma from the altar, placing it in the temple alongside images of living and deceased Augusti. This combination was unique in the Western Empire and the historian Fishwick interprets the reformed rites as those offered by slaves to a Roman paterfamilias.