When speaking of the Trinity, you can say three, but you can’t say triple or threefold or triplex. And you certainly shouldn’t say that the Father, Son, and Holy Spirit are three beings.
In these two axioms, Polanus is largely covering ground he has already covered early in the 18 Axioms, but he seems to do so mainly because he wants to make sure to mention a couple of things you shouldn’t say (triple and triplex), and a couple of things you should say (subsist, ενυποστατα). And as we should expect by now, Polanus is only willing to venture statements about threeness in the context of divine simplicty.
Here’s the translation:
Axiom 15. One cannot say, speaking substantively, that the Father, Son, and Holy Spirit are three beings; even though speaking adjectivally, one can say three beings subsisting, and likewise also three things, as also in Greek τρια ενυποστατα.
Axiom 16. God is three in persons, but not triplex: the former pertains to God, while the latter does not in any fashion. Just as he is simple without complexity, so also triplex denotes a complex of three, and likewise so does triple. Due to this, Augustine asserts that God can be spoken of as three and Trinity, but not at all as triple. Therefore, we avow that God is absolutely simple, i.e. devoid of any composition according to those things which are said to be principles; but we avow he is three according to procession or persons, all the while leaving the simplicity fully intact.
And here’s the conversation. It’s a short episode, and near the end there are a few moments where the internet’s laggy and the speech is draggy, but it’s all fairly clear:
Just one quick show note: I mentioned Benjamin Gleede’s book The Development of the Term ἐνυπόστατος From Origen To John of Damascus (Brill, 2012), which is an excellent discussion of this key theological word. The word is best known in recent theology as an element of Christology, and Gleede certainly covers that. But first he gives a solid forty pages to ἐνυπόστατος as a Trinitarian term. That’s the really useful part for our discussion of Polanus.