You do know that there is ALOT of difference between the Third Century and the Fifth Century, right?
The Third Century was the time of the Crisis of the Third Century, where Emperor Alexander Severus assassinated by fis own troops, initiating a fifty-year period in which 20–25 claimants to the title of Emperor, mostly prominent Roman Army generals, assumed imperial power over all or part of the Empire. By 258–260, the Empire split into three competing states: the Gallic Empire, including the Roman provinces of Gaul, Britannia and Hispania; the Palmyrene Empire, including the eastern provinces of Syria Palaestina and Aegyptus; and the Italian-centered and independent Roman Empire, proper, between them. The Crisis ended with the ascension and reforms of Diocletian, although it was his predecessor Aurelian who reunited the empire. There was also the Sassanid Dynasty to the East.
Meanwhile, the Fifth Century is where you're focusing, the decline of the Roman Empire, when barbaric tribes began to settle in ROman lands, when Atilla the Hun destroyed a huge chunk of the Roman Empire.