[Let me take a blog to reconsider this metaphor of a list with two columns: transcendence and immanence.
Under immanence, I write “the Father and the Son”. Why? Both appear to be actual. Secondness contains two elements. Both elements are actual. One element accounts for the other.
Under transcendence, I write “the Holy Spirit”. The Holy Spirit goes with thirdness because it expresses the relation binding the Father to the Son.
This relation goes with the mystery of God. The Holy Spirit3 brings “the Father and Son2” into relation with “the Omnipotence of God1”.]