Summarizing the Third Century Crisis

[jho]

Increasing pressure on the Roman frontiers led to the creation of new legions: Matcus Aurelius added two, Septimius Severus three. This could not be financed (Roman Empire had no economic growth), so they debased the denarius. Because the soldiers realized they were cheated, they demanded more money; and this created a spiral of inflation, which made an adequate defense very difficult.

It has been suggested that inflation was actually fairly modest (<4% p.a.) up to the era of Aurelian (cf. Inflation and Financial Policy under the Roman Empire to the Price Edict of 301, Alfred Wasink, Historia, Volume 40 / 1991), therefore inflation as such may not have played a major role in the crisis.