Flavius Valentinianus II, half-brother of Gratian, was made Augustus in AD 375 and was responsible for affairs in the West, after the fall of Maximus in AD 388, Valentinian took charge of Gaul. In AD 392 he was assassinated by orders of the German general Arbogast.
The AE3 above was minted in the period AD 378 - 383 when Gratian, Valentinian and Theodosius ruled - the very common reverse type struck at the time shows a helmeted and seated Roma with the legend CONCORDIA AVGGG (the most common other reverse type for the period displayed a turreted Constantinopolis with the same legend).

Obv. Diademed, draped, cuirassed Valentinian II right, D. N. VALENTINIANVS P. F. AVG.
Rev. Roma seated, holding globe and spear, CONCORDIA AVGGG.