Some heretics quote Jn 17:3 as a unitarian prooftext. That raises an interesting exegetical question: Why does Jesus call the Father the "only true God"? Monos means "only one". Cf. Louw/Nida, Greek-English Lexicon of the NT, 58.50.
I'd say that serves as a divine title. For the NT (or Jesus) to say God is the "only one" harkens back to the Shema (Deut 6:4). On that view, the Only One is a synonym for Yahweh–the one true God.
That would also explain why, on the most likely construction, John calls Jesus the "one true God" in 1 Jn 5:20. As a divine title and synonym for Yahweh, it's equally applicable to the Father and the Son.
And in that connection, Jn 17:3 is a flashback to Jn 10:30. Arguably, Jn 10:30 is an allusion to the Shema. In the context of a debate over the identity of Jesus in relation to the identity of God, "one" would inevitably evoke the central Jewish confession (Deut 6:4). It functions as a synecdoche for Yahweh.
Finally, it's not coincidental that Jn 17:3 is stated in the context of Christ's comparison and contrast between the unity of Christians and the unity of the Father and Son. That picks up on Jn 10:30 and develops an analogy. However, it's carefully compartmentalized. Christians aren't one with the Father. Rather, Christians are one with each other, analogous to how the Father and Son are one with each other (17:11,22).
There's a mediated sense in which Christians are one with God. If the Son is directly one with the Father, while Christians are directly one with the Son, then Christians are indirectly one with the Father (or God).
But that's because the incarnate Son is a bridge between God and man. Christians aren't one with the Son in the same sense that the Son is one with the Father.