(Could the following:)

Deuteronomy 19:15 One witness shall not rise up against a man for any iniquity, or for any sin, in any sin that he sinneth: at the mouth of two witnesses, or at the mouth of three witnesses, shall the matter be established.

(Possibly be prefiguring this?)

I John 5:7 For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.