An analysis of the phrase "the only true God" in John 17:3:

αὕτη δὲ ἐστιν ἡ αἰώνιος ζωὴ ἵνα γινώσκωσιν σε τὸν μόνον ἀληθινὸν θεὸν καὶ ὃν ἀπέστειλας Ἰησοῦν Χριστόν.

This is eternal life, that they may know You, the only true God, and Jesus Christ whom You have sent.