I believe the Three-Headed Demon is Diablo because it seems the portrait of this Diablo is based from the novel, Divine Comedy. In Dante's Inferno, Satan (Diablo) is frozen in the center of a lake in the 9th level of Hell who is eternally chewing/torturing on three corrupted humans. Brutus and Cassius, Betrayers of Julius Caesar, and Judas, Betrayer of Jesus. They were consumed by Satan and became the three heads from within him. So I can safely say that Blizzard chose the right portrait for Diablo.
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