The three-bodied giant whose cattle Heracles stole was Geryon.
Geryon lived on Erytheia, a mysterious island in the far western sea, and owned a magnificent herd of red cattle. Ancient accounts vary in describing his body: Hesiod presents him as one figure with three heads, while later traditions often give him three bodies joined together. The name Geryon therefore became closely associated with triple-bodied monster imagery.
Obtaining his cattle was Heracles’ tenth labor. Heracles reached Erytheia using the Sun-god Helios’s golden cup, killed the two-headed watchdog Orthrus and the herdsman Eurytion, then confronted Geryon. In the best-known account, a Hydra-poisoned arrow pierced the giant’s forehead.
The return journey was nearly as difficult. Hera sent a gadfly that scattered the herd, and in Roman versions the thief Cacus stole some cattle near the future site of Rome. Geryon is not Orthrus or Cerberus: Orthrus guarded the herd, while Cerberus guarded Hades.