The Roman religion seems to have had three gods of the dead…Pluto, Dis (Dispater), and Orcus. Does anyone know what the differences were between these three and what they represented?
The older Roman Religion is more animistic than the anthropomorphic Greek gods; the gods were more about notions of divine power–numen is the word in Latin–rather than the deus concept borrowed from the Greeks.
Pluto as a name for the god of the dead in Roman religion is borrowed directly from the Greek word for “wealth”. Dis (Dispater) is most likely a shortened form of Dives Pater; wealth was often associated with the realm of the dead in the ancient world. Could also be short for Deus pater, the root for name Juppiter.
Orcus is primarily the place of the underworld rather than the god that rules there. The L&S entry at Perseus tells us this is akin to Greek herkos - “fence, wall” and the Latin urgeo - “compell, hem-in”. Certainly by the time of Plautus–when applied to the god–it was simply an alternate name for Pluto.