How do Trinitarians explain John 17:3? Unitarians use John 17:3 to affirm their faith that the Father alone is the only true God. They would say that this is the strongest evidence that there is only one person who is true God. Thus, the Nicene Creed saying that Jesus was "true God from true God" would appear to be contradictory to John 17:3 based on Unitarian perspective. 


  And this is eternal life, that they know you, the only true God, and Jesus Christ whom you have sent.
  John 17:3 (ESV) 


How do Trinitarians explain John 17:3? Can I have an answer with an elaboration on the passage itself, a patristic quotation and also with scholarly citation from Trinitarian perspective?
That scripture verse that you mention was used by Arius to maintain that Jesus was not of same nature as Father. However, his objection was resolved.

St. Thomas explains the difficulty you mention in his commentary on the gospel of John 17; on this point he writes:


  Secondly, we should explain the phrase, you the only true God. It is
  clear that Christ was speaking to the Father, so when he says, you the
  only true God, it seems that only God the Father is true God. The
  Arians agree with this, for they say that the Son differs by essence
  from the Father, since the Son is a created substance, although he
  shares in the divinity more perfectly and to a greater degree than do
  all other creatures. So much more that the Son is called God, but not
  the true God, because he is not God by nature, which only the Father
  is.
  
  Hilary answers this by saying that when we want to know whether a
  certain thing is true, we can determine it from two things: its nature
  and its power. For true gold is that which has the species of true
  gold; and we determine this if it acts like true gold. Therefore, if
  we maintain that the Son has the true nature of God, because the Son
  exercises the true activities of divinity, it is clear that the Son is
  true God. Now the Son does perform true works of divinity, for we
  read, "Whatever he [the Father] does, that the Son does likewise"
  (5:19); and again he said, "For as the Father has life in himself,"
  which is not a participated life, "so he has granted the Son also to
  have life in himself" (5:26); "That we may be in his true Son, Jesus
  Christ. This is the true God and eternal life" [1 Jn 5:20].
  
  According to Hilary, he says, you the only true God, in a way that
  does not exclude another. He does not say without qualification, you
  the only, but adds and Jesus Christ whom you have sent. It is like
  saying: that they know you and Jesus Christ whom you have sent to be
  the one and only true God. This is a pattern of speaking that we also
  use when we say [in the Gloria]: "You alone, Jesus Christ, are the
  most high, together with the Holy Spirit." No mention is made of the
  Holy Spirit because whenever the Father and the Son are mentioned, and
  especially in matters pertaining to the grandeur of the divinity, the
  Holy Spirit, who is the bond of the Father and Son, is implied.
  
  Or, according to Augustine in his work, The Trinity, he says this to
  exclude the error of those who claim that it is false to say that the
  Father is God, and the Son is God, and the Holy Spirit is God; while
  it is true to say that the Father and the Son and Holy Spirit are one
  God. The reason for this opinion was that the Apostle said that
  "Christ [is] the power of God and the Wisdom of God" (1 Cor 1:24). Now
  it is clear that we cannot call anyone God unless he has divine power
  and wisdom. Therefore, since these people held that the Father was
  wisdom, which is the Son, they held further that the Father considered
  without the Son would not be God. And the same applies to the Son and
  the Holy Spirit.


Also in Summa Contra Gentiles St. Thomas Aquians sets up various objections (that Arius used) against opinion that Jesus is God here (among objections is one that uses John 17:3.) He resolves them here. It is benifical to read objections and answers for detailed solutions to the objections.

St. Thomas argues that in God there is begetting, fatherhood and sonship in book IV of Summa contra gentiles (see chapters from 1 to 14) where he also refutes opinions of Protinus and Sabellius.