The problem that Christians have when confronted by the JWs on this teaching, is that as the JWs themselves point out, there is no explicit teaching of the Trinity in the Bible. The JWs then wrongly conclude that because so-called "pagans" believed a teaching remotely descriptive of it, it therefore must be of ''pagan ORIGIN" We can with equal validity conclude that because these "Pagans" believed in the flood, that the flood must also be a teaching with a pagan origin. Indulgence never determines origin, and when the WTS asserts this, they merely indicate their own lack of scholarship.
The point is that no doctrine is explicitly taught in Scripture. This is because the thrust of Biblical teaching is to encourage personal investigation, so doctrine is tightly bound up in statements implicity imbedded in the Bible and capable only of individual scrutiny and conclusion. Indeed none of the the doctrines so beloved by the WTS is explicitly taught in the Bible, even in their own "Translation". The "last generation" teaching as expounded by the WTS has had to be modified at least on six seperate occasions, thus deceiving six seperate generations, precisely because it is not an explicit Bible teaching. The summation of WTS doctrine is arrived at by simply stitching together randomly collected verses and subjecting these to relentless interpretation.
It is true that the OT does not provide us with an incontroverible clue that affirms the existence of a Triune Deity, nevertheless, nowhere does it actually deny such a teaching which would later develop from the revelation of NT Scripture. The main contribution of the OT to the doctrine is to emphasize the unity of God. God is Himself not a plurality, nor is He one among others [Henotheism] God is revealed in the OT as single and unique. He is not made up of "parts" as if He were a three-in-one entity.
In the fully developed doctrine of the Trinity, arrived at by extensive and incremental study of the NT, especially over the first 4 centuries of the historic Christian Church, the unity of God is safeguarded by insisting that there is only one "essence" or "substance" of God. The Deity of Christ is fully asserted, because of revelation, and the individual existence of the Father and the Holy Spirit is preserved against the notion that these are only modes of God for the various purposes of dealing with man in creation and salvation.
Therefore the Trinity teaches that God is One [never three-in-one] yet Himself , and from all eternity is Father, Son, and Holy Spirit. They are three and at the same time one. To deflect any suggestion of a three-in-one Deity, the Trinity explicitly states that there be no ''confounding of the Persons, nor any dividing of the Substance" The Christian admits to the fact the Trinity yeilds to no adequate explanation, and that God inhabits a dimension beyond human reasoning, defying explicability.
When the early Christians, conscious of the clear implication of NT revelation, began their hitherto unexplored quest for the meaning of God, they never once denied the plain assertion of the "Sheema" of Lev 6:6, they merely extended its meaning and further developed it. Perhaps in a mysterious verse like Isa 48:16, in a strongly monotheistic context, we have a close approach to trinitarian formulation.
Cheers