The following translation/paraphrase of Galatians 3:20 from the Amplified Bible, Classic Edition, is quite amazing:
“There can be no mediator with just one Person. Yet God is only one Person. He was the sole party in giving that promise to Abraham” (AMPC).
I call it amazing because most Christians have been taught that the one God of the Bible is not “only one Person,” i.e., the Father, but “the Father, the Son and the Holy Spirit”–the Trinity!
Now the reason the AMPC has rendered the Greek phrase in Gal 3.20 (theos eis) as “one Person” is that whenever the Greek word for “one” (eis) appears with the word “God” or “Lord” or the Divine Name Yahweh, it is the equivalent in English of “one person.” This, of course, is confirmed by thousands and thousands of singular personal pronouns used for God throughout both the Old and New Testament Bible.