I'll simply offer you what comes to my mind, and a few sources that I would consult with respect to patristic exegesis of John's Gospel with respect to the Trinity.
By all means, I would consult...
1) Douglas F. Kelly, Systematic Theology, Vol. 1, The God Who Is: The Holy Trinity (Fearn, Scotland: Christian Focus Publications, 2008). No other Reformed systematic theologian interacts as much as Kelly does with respect to patristic sources pertaining to the Trinity. He is a "must" to consult and read on this topic.
2) R. P. C. Hanson, The Search for the Christian Doctrine of God: The Arian Controversy 318-381 (Edinburgh: T & T Clark, 1988). Although Hanson is not a Reformed theologian, his work is the best and most exhaustive work, from an historical perspective, on this period of the early church. He addresses differences in exegesis between the Orthodox and the Arians with respect to specific exegetical questions, including the Gospel of John.
3) Of course, you will want to consult (just a few recommendations) Augustine's Tractates on the Gospel of John and Cyril of Alexandria's Commentary on John in two volumes and can be downloaded from the internet in a pdf format. You will also want to consult Origen's Commentary on John to investigate the seeds he sowed which in turn helped provoke the Arian controversy.
One of my favorite quotes from Augustine, to paraphrase him, is "Go to the Jordan (namely the Gospel accounts), and there learn the doctrine of the Trinity" as deduced from the quote below...
Augustine (354-430): The lesson of the Gospel hath set before me a subject whereof to speak to you, beloved, as though by the Lord’s command, and by His command in very deed. For my heart hath waited for an order as it were from Him to speak, that I might understand thereby that it is His wish that I should speak on that which He hath also willed should be read to you. Let your zeal and devotion then give ear, and before the Lord our God Himself aid ye my labor. For we behold and see as it were in a divine spectacle exhibited to us, the notice of our God in Trinity, Conveyed to us at the river Jordan. For when Jesus came and was baptized by John, the Lord by His servant (and this He did for an example of humility; for He showeth that in this same humility is righteousness fulfilled, when as John said to Him, “I have need to be baptized of Thee, and comest Thou to me?” He answered, “Suffer it to be so now, that all righteousness may be fulfilled”), when He was baptized then, the heavens were opened, and the Holy Spirit came down upon Him in the form of a Dove: and then a Voice from on high followed, “This is My beloved Son, in whom I am well pleased.” Here then we have the Trinity in a certain sort distinguished. The Father in the Voice, — the Son in the Man, — the Holy Spirit in the Dove. It was only needful just to mention this, for most obvious is it to see. For the notice of the Trinity is here conveyed to us plainly and without leaving room for doubt or hesitation. NPNF1: Vol. VI, Sermons on Selected Lessons of the New Testament, Sermon 2, §1.
