Short answers, pulled from the story.
The Crisis of the Third Century was a period of Roman history, usually dated from 235 to 284, during which the empire nearly collapsed under repeated foreign invasions, civil wars, and economic disintegration. At its worst, the Roman state split into three competing polities: the Gallic Empire in the west, the Palmyrene Empire in the east, and the Italian-centered Roman Empire proper.
The crisis began in 235 with the assassination of Emperor Severus Alexander by his own troops at Mogontiacum in Germania Superior. The soldiers then proclaimed Maximinus Thrax, a military commander of humble origins, as emperor, bypassing the Roman Senate entirely.
Over fifty people claimed the title of augustus during the crisis period. Of these, about half received recognition from the Roman Senate and became legitimate emperors. With very few exceptions, every legitimate emperor during this period died violently, most killed by their own troops.
Emperor Aurelian (270-275) reunited the empire by defeating the Palmyrene Empire and the Gallic Empire, with the empire fully reunited by late 274. The crisis is said to have formally ended with Diocletian, who reorganized the Roman imperial government, economy, and military and established the Tetrarchy of four co-emperors in 293.
Decades of currency debasement, as each new emperor inflated the coinage to pay military accession bonuses, caused hyperinflation that nearly destroyed the denarius by the end of the third century. Civil unrest made Rome's extensive trade network unsafe for merchants, forcing large landowners to shift from cash-crop export to local subsistence production, a transformation that the source identifies as a forerunner of medieval manorialism.
The Plague of Cyprian, possibly smallpox, devastated the Roman Empire from 249 to 262. The city of Alexandria alone experienced a 62% decline in population during this period. The plague weakened the army's ability to resist barbarian invasions, contributed to famine by causing farms to be abandoned, and deepened the political instability already caused by civil war.
The Gallic Empire was a breakaway state that split from Rome in 260 and lasted until 274. It controlled the western provinces of Hispania, Gaul, and Britannia, had its own capital at Trier, its own succession of emperors, and its own yearly-elected consuls. It was defeated and reunited with the Roman Empire by Aurelian.