Ubi notandum est quod natura non se habet ad suppositum sicut universale ad singulare, quia in accidentibus etiam invenitur singularitas sine ratione suppositi, et in substantia nostra natura atoma assumpta est a Verbo, secundum Damascenum, non tamen suppositum nostrae naturae. Neque se habet natura ad suppositum sicut 'quo' ad 'quod', nam cuicumque 'quo' correspondet proprium 'quod' vel 'quis', ita habet proprium 'quod' vel 'quis' quod non contrahit ad suppositum, et sicut suppositum est 'quod' vel 'quis', ita habet suum proprium 'quo' quo subsistit et tamen concomitanter suppositum de necessitate est singulare,--et etiam, non potest esse 'quo' respectu alterius, quia est subsistens, non potens esse actus alicuius subsistentis; haec duo dicunt duplicem incommunicabilitatem.
"Whence it must be noted that the nature is not related to a supposit as universal to singular, since singularity is found even in accidents without them being a supposit [sine ratione suppositi], and in our substance an atomic [individual] nature is assumed by the Word, according to the Damascene, but not, however, a supposit of our nature. Neither is nature related to supposit as 'quo' to 'quod', for to any 'quo' [by which] corresponds its own 'quod' [what] or 'quis' [who], and so, as the nature is the 'quo', so it has its own 'quod' or 'quis' which does not contract to a supposit, and as a supposit is 'quod' or 'quis', so it has its own 'quo' by which it subsists and nevertheless at the same time [concomitanter] a supposit is necessarily singular;--and also, it is not able to be 'quo' with respect to another, for it is subsistent, not able to be the act of some [other] subsistent; these two bespeak a double incommunicability."
--Ordinatio I dist. 2 pars 2 Q. 1-4 para.378.
The Divine Nature which the three Persons share is not abstract but concrete and singular, as are each of the three Persons themselves. This Nature is not "prior" to the supposits, nor does it act, for only supposits act. The Persons act through their Nature; but the Nature is not without qualification absolutely identical to any or all of the Persons, for if it were identical to, say, the Father, it could not be identical to the Son without contradiction. But it is certainly not something other than the Persons. Hence the need for the formal distinction--matter for a later post.