Here is an article by Jews for Jesus reasoning for the Trinity from the Old Testament (Hebrew Bible) A Jewish View of the Trinity Based on the Hebrew Scriptures
I’ve read a number of books on this topic, and this article from Jews for Jesus had some good arguments I hadn’t heard. It’s a good apologetic for the Trinity being in the Old Testament.
It quotes numerous scriptures and discusses the names for God, pronouns, plural nouns with singular verbs for God, plural nouns with plural nouns for God, different Hebrew words for one, and many other interesting points. It is done in a way that is easy to read.
One thing I found interesting was the examples of God having both a plural noun and plural verb. Usually in the Old Testament (Hebrew Scriptures)God is in the plural with a singular verb. This article gave some exceptions along with noting the more standard usage.
This article should be good as a resource for Jewish evangelism. It should also be useful in talking with other people who deny the Trinity like Mormons, Jehovah’s Witnesses, and Muslims.
You can find posts like this on our website under Jewish Evangelism