One god under three names, across The Olympian Order, The Etruscan World and The Roman Inheritance.
Hadeshay-deezLord of the Dead — king of the underworldThe Olympian OrderAitaai-taThe Etruscan Lord of the Underworld — equivalent to Greek HadesThe Etruscan WorldDis Paterdispa-terThe Rich Father — Roman lord of the underworldThe Roman Inheritance
Why the sources call them one god
His name is the Etruscan form of the Greek Aides (Hades)