It would seem that not only is Potamius' choice of "three tents" for Moses, Elijah, Jesus, a poor choice theologically (from a literal text meaning point of view), but also it appears that the choice of Mark 9:5 (9:4 Latin versions) by Potamius might have been a poor choice
grammatically if he was trying to get it to agree (i.e. being in grammatical concord) with 1 John 5:7, because the words for both "three" and "tents" and "one" (as in "one for you" speaking to Jesus) are in
the feminine gender in the Greek (the language Mark the Gospel writer originally wrote in), which, I'm sure you would agree Steven,
the feminine gender is totally unsuited to the Trinity.
Mark 9:5
καὶ ἀποκριθεὶς ὁ Πέτρος λέγει τῷ Ἰησοῦ Ῥαββεί, καλόν ἐστιν ἡμᾶς ὧδε εἶναι, καὶ ποιήσωμεν
τρεῖς [= adjective, accusative, feminine, plural] σκηνάς [= noun, accusative, feminine, plural], σοὶ
μίαν [= adjective, accusative, feminine, singular] καὶ Μωυσεῖ μίαν καὶ Ἠλείᾳ μίαν.
But, considering
Potamius was into Mary worship,
he doesn't appear to be bothered by this possible grammatical
solecism.
Note how he ends his letter to Athanasius (of all people) with
a doxology addressed to Mary as "the Mother of God" along with the Trinity.
Potamius of Lisbon. "Letter to Athanasius the Bishop of Alexandria on the consubstantiality of the Son of God" in the life and works of Potamius of Lisbon edited and translated by Marco Conti, 1998, p. 140-141.
[Page 141] Latin Text: "Obmutescat heresis aeterno silentio praedamnata, divinis ictibus caesa, baratro tartarisque deposita. Sola semper cum laureis suis virgo puerpera Deo, una nobis columba, fecunditate numerosior, usque ad nubes caeli caput coronatum adtollat. Sit benedicta cum populis, Trinitatis unitate consecrata iustissimis, cuius laude plena est a nunc et in aeterna semper saecula saeculorum. Beatificet Pater et Filius et Spiritus sanctus."
[Page 140] "May the heretical sect be suppressed, after being condemned to eternal silence, smashed by the lightning of God and forsaken in the abyss of hell.
May the Virgin alone, Mother of God, raise to the clouds of heaven her head crowned with laurels, she who is our only dove and is superior in fecundity. May she be blessed among the nations who are piously faithful to the sacred unity of the Trinity
and fully praise her now and forever, world without end. May the Father, the Son and the Holy Spirit make you blessed."