To this it must be said, that in eight ways the plurality of the Persons is insinuated to us in Scripture. In the first way by signification in the last (chapter) of Matthew (Mt 28:19): In the Name of the Father and of the Son and of the Holy Spirit. In the second way by consignification at the beginning of Genesis, where we have "God", the Hebrews have "Elohim", which is the nominative plural of the singular "El". In the third way by supposition, as when it is said God begot God in the eighth (chapter) of Proverbs: Before all the hills the Lord has generated Me (Pr 8:25). In the fourth way by appropriation, as so (Gn 1:1): In the beginning God created etc. For there "God" is appropriated to the Father and The Beginning to the Son. In the fifth way by iteration, as in the sixth (chapter) of Isaiah (Is 6:3): Holy, Holy, Holy the Lord God Sabaoth. In the sixth way by the order of words, (as in) the Psalm (Ps 67:6-7): May God bless us, our God, may God bless us. In the seventh way by connotation in the act of mission, as when there is said in the fourth (chapter) to the Galatians (Gal 4:4): God sent etc. In the eighth way by apparition, just as the three men appeared to Abraham in the eighteenth (chapter) of Genesis.
St. Bonaventure, Commentary on the Sentences, bk. 1 d. 2 dub 8