Passage 2.24.4
τρεῖς δὲ ὀφθαλμοὺς ἔχειν ἐπὶ τῷδε ἄν τις τεκμαίροιτο αὐτόν. Δία γὰρ ἐν οὐρανῷ βασιλεύειν, οὗτος μὲν λόγος κοινὸς πάντων ἐστὶν ἀνθρώπων. ὃν δὲ ἄρχειν φασὶν ὑπὸ γῆς, ἔστιν ἔπος τῶν Ὁμήρου Δία ὀνομάζον καὶ τοῦτον· Ζεύς τε καταχθόνιος καὶ ἐπαινὴ Περσεφόνεια. Hom. ll. 9.457 Αἰσχύλος δὲ ὁ Εὐφορίωνος καλεῖ Δία καὶ τὸν ἐν θαλάσσῃ. τρισὶν οὖν ὁρῶντα ἐποίησεν ὀφθαλμοῖς ὅστις δὴ ἦν ὁ ποιήσας, ἅτε ἐν ταῖς τρισὶ ταῖς λεγομέναις λήξεσιν ἄρχοντα τὸν αὐτὸν τοῦτον θεόν.
One might conjecture that he was depicted with three eyes for the following reason. That Zeus reigns in heaven is belief commonly held by all mankind. A Homeric verse applies the name of Zeus also to him whom they say rules beneath the earth: "the underworld Zeus and dread Persephone" (Il. 9.457). Aeschylus, son of Euphorion, calls Zeus also lord of the sea. Therefore whoever fashioned this god gave him three eyes, because the same deity rules the three realms traditionally spoken of.