My point is that to claim that the Old Testament teaches the doctrine of the Holy Trinity is a bit absurd. Can we say there were hints and prefigures? Certainly, but did the authors even understand the text in that way?
Well, I agree that the OT authors probably did not understand the text in that way. But surely it is a leap of Judaic proportions to conclude that the OT does not teach the doctrine of the Trinity.
In Acts 2.14ff, St Peter cites the prophecy of Joel to explain what was happening before the Jewish people on Pentecost. Did the prophet make his prophecy explicitly knowing and teaching that it referred to the descent of the Holy Spirit on the apostles of the second Person of the Holy Trinity in the upper room of the house of Mark in Jerusalem during the high priesthood of Annas and Caiaphas when Pontius Pilate was governor of Judea? Probably not. But that doesn't stop St Peter from saying this was definitely what Joel was talking about.
As Christians, and especially as Orthodox (and RC's too, I should think), we ought not make too much of a dichotomy between the OT and the NT, as if they're two separate things. They are to the Jews, but we're not Jewish for a reason. In the light of Christ, we can see and understand the true meaning of the OT Scriptures and what they were teaching all along. In Christ, as St Paul teaches, the veil is removed (cf. II Cor. 3.12-18). There's no need to put it back on.