There are certainly OT passages which _in the light of the NT_ suggest
incarnational & thus trinitarian motifs - e.g., Zech.12:10 (where the Hebrew reads "they
shall look on me whom they have pierced"). I find something like this much more
significant than any numerical hints of plurality &c. The basis for all Christian
trinitarian thought is the incarnation & cross. Without that, any supposed hints of
plurality in God are of no value.
& it is essential to emphasize that this is all a result of reading the OT in
light of the new. It's a complete mistake to suggest to Jews that they should be able
to find suggestions of the Trinity in OT passages read on their own terms.
George L. Murphy