Sons of Constantine
Emperor Constantine divides the Empire amongst his three sons (as in OTL, but not with Constantine II and Constans sharing the West).
(Eldest) Caesar Constantine II establishes his capital in Massilia.
(Middle) Caesar Constantius takes the throne in Constantinople.
(Youngest) Caesar Constans is crowned in Alexandria.
Augustus Constantine retains the city of Rome, which he decrees now encompasses all of Italia and Sicily.
West Rome - Capital Massilia; Iberia, Gaul, Belgica, Britannia, Germanias, Illyricum
East Rome - Capital Constantinople; Greece, Macedonia, Moesias, Thracia, Anatolia (et al), Syria, Judea
South Rome - Capital Alexadria; Egypt, Africa, Mauretanias, Cyrenacia, Nubia, Arabias
Imperium Civitas Roma - Italy, Sicily