Richard Muller notes three characteristics of Reformed Orthodoxy's discussion of the doctrine of the Trinity: First, they showed a careful appropriation and development of Patristic vocabulary; second, they demonstrated a clear appreciation of the exegetical ground of the doctrine in Scripture; and third, they struggled to find philosophical categories for the expression of the doctrine given the increasingly problematic conception of the term "substance" in the seventeenth century [Post-Reformation Reformed Dogmatics 4:58-62].
Given the degree of anti-Trinitarian sentiments in both the sixteenth and seventeenth centuries (Socinianism in particular)*, and as Trinitarian statements go, the WCF's two sentence, fifty-five word statement is profoundly terse and even simple. We should not conclude, however, that the Divines minimized the doctrine; on the contrary, any form of Unitarian theology was viewed as heresy and, more especially, un-Christian and even anti-Christian. One-in-threeness and Three-in-Oneness constitutes orthodox theology and the Divines bow to Tertullianesque, Niceno-Constantinopolitan creedal justification for the statements they make. There is One God but there is also MORE THAN ONE who is the ONE God. The Father is God. The Son is God and the Holy Spirit is God.
Truth is, God can only exist this way. Truth is, too, that the gospel can only be the way that it is because God is triune, for love - out-going, sacrificial love that we see in Christ's Mediatorship - can only exist because of plurality within the unity of God. Without Trinity there is no possibility of gospel.
[* See, Paul C. H. Lim, The Crisis of the Trinity in Early Modern England (OUP, 2012); Philip Dixon, Nice Hot Disputes: The Doctrine of the Trinity in the Seventeenth Century (T & T Clark, 2006)]