If I could provide you with links to a nice translation of Amandus Polanus’ Axioms on the Trinity within his Syntagma Theologiae, I would. But I can’t. What Ryan Hurd and I are doing in this set of conversations is posting Ryan’s new translation of each axiom, then talking about it.
So here’s the second axiom, which we might entitle “Trinity in Person, not in Essence.”
Axiom 2. There is Trinity not in essence, which is single, but in the number of persons and the distinction of them. This is akin to what Tertullian says: the unity of the Monarchy is the Trinity of the economy, and the one does not destroy the other. Likewise, Thomas Aquinas says: “When we say Trinity in unity, we do not impose number into the unity of essence, as if there was three times one; rather, we intend the numbered persons in the unity of nature.” Therefore, this affirmation works both ways, “The three persons are that single divine essence,” because it is clear that the divine essence is not broader than the actual three persons. And what is more: just as one does not say God is multiplex or triplex, because he is absolutely simple under the concept of essence, but instead say he is triple under the concept of the persons—so likewise one does not say he is the multiplicity or triplicity but the Trinity of persons, lest one think of more divine essences.
 Contra Praxean 2 and 3.
 Summa Theol I:31, art 1 ad 4