Constantine III (the usurper) was an ordinary foot soldier in Britain when he rallied some troops, moved into Gaul, fought a few battles and had himself declared Emperor. He was not related to anyone in the Constantinian Dynasty. and was killed in 411.
Ultimately he was brought down by Constantius III, magister militum under Honorius who married Galla Placida after her return from Visigothic Spain in 417. They had two children, Just a Grata Honoria and Valentinian III in in 417 and 419. He died in 421.
Neither of these two parties had a daughter named Magna Major or Magna Constantia born c 370 who supposedly married Flavius Julius Agricola-nor is it even possible that a distinguished Senator and Praetorian Prefect and Consul of Rome would entertain the notion of marrying relative of (a) a usurper to the throne or (b) the sister of a common foot soldier, who, moreover is not recorded as having a daughter by that name.
This Magna Major is not the mother of Roman Emperor Avitus, nor the wife of Flavius Julius Agricola although there are several misguided websites that promote this fairy tale: Fab pedigree, genealogies online, my heritage (where you can find almost anything, but nothing accurate!
Wikipedia has very good write ups for both of the above candidates.search Flavius Claudius Constantius, usurper and or Constantius III d 421.
