According to Catholicism, did Moses, and especially the Jewish people during his time, know that God is triune?
St. Thomas Aquinas explains this in his question on "Whether it is necessary for salvation to believe explicitly in the Trinity?" (Summa Theologica II-II q. 2 a. 8):
…before Christ, the mystery of Christ was believed explicitly by the learned, but implicitly and under a veil, so to speak, by the simple, so too was it with the mystery of the Trinity.
Moses was certainly among "the learned".
My Catholic scriptural scholar friend wrote me:
Moses was indeed instructed in the mystical science and (literally) hieroglyphic [ἱερός = sacred, holy] sign-ification of the tetragrammatic but Trinitarian Name of the only true God, יהוה (Y-H-W-H).
The inspired description of the divine essence as relationality was given him to know in the properly heard revelation recorded in Exodus 3:14: “I’m being/I’m/I shall be who I’m being/I’m/I shall be” → an a-temporal ACTIVE present: אֶהְיֶה.
Thus, St. Moses did know, albeit in the mystical darkness of the cloud on Mount Sinai, that Y-H-W-H is He whose essence is to be itself (ipsum esse) as relation; He, that is, whose essence is to be relation to Himself—i.e. of knowing and loving Himself. Verse 14 tells us who this God is in terms of His essence, which is ‘to be’ itself as r-e-l-a-t-i-o-n → which the specific Hebraic relative pronoun is clearly intended to emphasize, אֲשֶׁר, as it connects (and thereby also differentiates) the mirroring/symmetrical clauses אֶהְיֶה (“I’m being/I’m/I shall be”) grammatically designating God’s indivisible esse:
“I’m being/I’m/I shall be [אֶהְיֶה] who [אֲשֶׁר] I’m being/I’m/I shall be [אֶהְיֶה].”
St. Abraham knew the triune God under a certain mystical mode of His blessed revelation (a true visitation and vision, see Gen 18:2), as he was visited by “three men” (אֲנָשִׁים), i.e. three subsisting personalities who are the One divine essence, whom he unmistakably addressed as: “My Lord…” (Gen 18:3).
He was the recipient of other revelations of the triune God (as were Moses and other holy Patriarchs), having seen and rejoiced in the day of the Only-Begotten Son (John 8:56) → in other words (dixit St. Thomas in the section and § you referenced to [above]), in mysterio Christi.
In the Old Testament the Trinity of Persons is expressed in many ways; thus at the very outset of Genesis it is written in manifestation of the Trinity: "Let us* make man to Our* image and likeness" (Gn. 1:26).
*"Us" and "our" are first-person plural; ∴, there are multiple Divine Persons.