Here is one that looks like the orthodox flip-side to the Arians singing against the three are one!
Marius Victorinus
or .. (Gaius) Marius Victorinus (Afer)
https://en.wikipedia.org/wiki/Gaius_Marius_Victorinus
Gaius Marius Victorinus (also known as
Victorinus Afer; fl. 4th century) was a Roman grammarian, rhetorician and Neoplatonic philosopher. Victorinus was African by birth and experienced the height of his career during the reign of Constantius II. He is also known for translating two of Aristotle's books from ancient Greek into Latin: the Categories and On Interpretation (De Interpretatione).[1] Victorinus had a religious conversion, from being a pagan to a Christian, "at an advanced old age" (c. 355).
The Witness of God is Greater
Hymn]
The three are therefore one,
And three times over,
Thrice are the three one,
O Blessed Trinity.
o Tres ergo unum,
et ergo ter,
ergo ter tres unum:
o beata Trinitas.
• Marii Victorini Afri, Hymnus 3, Migne Latina, PL 8.1146
The Epistles and Gospels of the Sundays Throughout the Year, Volume 2 (1866)
Daniel McCarthy
https://books.google.com/books?id=SuxS-z-6SIUC&pg=PA516
Marius Victorinus has additional writings related to the heavenly witnesses verse, writing against the Arians and in his Commentary on Philippians. Grantley Robert McDonald gave him some note in
Raising the Ghost of Arius, The Witness of God is Greater has good material and I am adding some from the verse debate history.
A good research study would be to see if there are additional flip-side hymns that were used to counter the Arians.
Bengel mentions the hymn here, in his superb section about heavenly witnesses evidences.
D. Io. Alberti Bengelii Apparatus criticus ad Novum Testamentum: criseos sacrae compendium, limam, supplementum ac fructum exhibens (1763)
https://books.google.com/books?id=N9w-AAAAcAAJ&pg=RA2-PA462
D Christiani Friderici Schmidii ... Historia antiqua et vindicatio canonis sacri Veteris Novique Testamenti (1775)
Christian Friedrich Schmid
http://books.google.com/books?id=d2IUAAAAQAAJ&pg=PA555
Marius Victorinus is in the heavenly witnesses verse debate involving Michaelis, Porson, Burgess, Turton and others, although off-hand I am not sure if they are mentioning the hymn.