With regard to the revelation of the Trinity in the Old Testament Turretin puts forward the following (Third Topic: Question 26):
Can the mystery of the Trinity be proved from the Old Testament, and was it known under it? We affirm against the SociniansTurretin argues that the Trinity is a fundamental article. The Socinians claimed that it was invented it as a new doctrine after the time of Christ. It is therefore incumbent on the orthodox to establish the "truth of this mystery not only from the New, but also from the Old Testament."
Here are some highlights:
Indeed, we confess that it was not revealed under the Old Testament with the same clearness as it is now taught in the New...Yet this is no objection to its having been made known even to the patriarchs sufficiently for salvation."
"...there is not one Old Testament and another New Testament God...but one and the same revealed in both as the sole object of faith and worship. Under the New Testament, he has revealed himself as one in essence and three in persons.
Therefore he must necessarily have been revealed to the Jews as such and known and worshipped by them. Otherwise they would not have worshipped the true God who is no other then the Father, the Son and the Holy Spirit (because he who has not the Son has not the Father either, 1 Jn. 2:23)."
Finally, if the Trinity was not revealed in the Old Testament, the orthodox thus far (ancient as well as the more modern) have labored falsely to prove it from the Old Testament (which cannot without grievous injustice be charged upon so many great men and faithful servants of God). Nor ought it to be said that we can now indeed gather this from the Old Testament assisted by the light of the gospel; but that it could not be done equally by the fathers [patriarchs].
For although we confess that the light of the New Testament serves in a great measure to illuminate for us the obscurity of the ancient oracles, yet it cannot be denied that God, who condescended to reveal them to the fathers for their instruction and consolation, adapted them to their comprehension so that they might from them be instructed in this mystery (as far as was necessary for their salvation). Otherwise to no purpose were these things revealed to them.