Mike Bird talks about this on his blog. Here is conclusion:
In my mind, the Trinity is both a biblically analytic doctrine (based on the raw data of Scripture) and also a theologically synthetic doctrine (based on inferences drawn from Scripture). In other words, Scriptures gives us “biblical pressure” (Kavin Rowe’s term) to construct a Trinitarian doctrine even if it does not explicitly lay out that doctrine. That is because of the triadic nature of the economies of creation and redemption which include Father, Son, and Holy Spirit and because of scriptural affirmations about the personhood and deity of the Father, Son, and Spirit which are also affirmed in Scripture. The Trinity is the conceptual model we use to make sense of the biblical materials and to show their theological coherence. While classic Trinitarians statements in some sense go beyond the New Testament, without the Trinity we have a hard time making sense of what the New Testament says about the Father, Son, and Holy Spirit.