In his Tractates on the Gospel of John, Augustine (354-430 AD) writes the following regarding John 17:3:
Augustine - Tractate 105 on the Gospel of John
3 "And this", He adds, "is eternal life, that they may know You, the only true God, and Jesus Christ, whom You have sent". The proper order of the words is, "That they may know You and Jesus Christ, whom You have sent, as the only true God". Consequently, therefore, the Holy Spirit is also understood, because He is the Spirit of the Father and Son, as the substantial and consubstantial love of both. For the Father and Son are not two Gods, nor are the Father and Son and Holy Spirit three Gods; but the Trinity itself is the one only true God. And yet the Father is not the same as the Son, nor the Son the same as the Father, nor the Holy Spirit the same as the Father and the Son; for the Father and Son and Holy Spirit are three [persons], yet the Trinity itself is one God.
Above, apparently sensing the tension that a straightforward reading of John 17:3 creates for the doctrine of the Trinity, Augustine modifies the verse so that instead of Jesus calling the Father "the only true God", He instead is made to call Himself and the Father "the only true God". Then, Augustine reads in the unmentioned Holy Spirit as well, and therefore derives the Trinity from the passage.
However, in the passage in question, when it is left as it was originally written, Jesus is distinguishing Himself from "the only true God", and saying that He is "sent" by Him. This demonstrates that the author of the Gospel of John was not confused as to who the one true God is, and that he believed Him to be the Father alone. And, this verse gives context from which to interpret the other statements in his Gospel that are commonly thought to teach that Jesus is God, and shows that he did not believe that.
Even though what Augustine did in this work was inappropriate, because he changed the words of the Bible to match his theology, it is worth acknowledging the fact that he at least saw the difficulty that the unaltered text created for his theology. Instead of doing what he did, however, it would have been better to reconsider his theology in light of the reading of the passage, instead of altering the reading of the passage to match his theology.