Commentaries on Boethius (1966)
Nikolaus Haring
https://books.google.com/books?id=dCIs4UeMXGgC&pg=PA113
see p. 113 footnote .. search "tres sunt"
"Tres unum sunt" - Boethius und Anselm von Canterbury über die philosophische Basis der Trinitätslehre (2005)
Joseph Kormos
http://coloman.viola.pagesperso-orange.fr/Anselm_Conference_Hungary.html
Boethius as a Paradigm of Late Ancient Thought (2014)
https://books.google.com/books?id=jcDoBQAAQBAJ&pg=PT341#v=snippet&q="tres unum"&f=false
The Autonomy of History: Truth and Method from Erasmus to Gibbon (1999)
by Joseph M. Levine
https://books.google.com/books?id=tIEGBimKQSwC&pg=RA1-PA46
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Anitii Manlii Severini Boethii, Philosophorvm et Theologorvm Principis Opera omnia, ...: Praeterea iam accesservnt Ioannis Murmelij in v.Lib. De Consolatione Philosophiae commentaria. Et in eosdem Rodolphi Agricolae Enarrationes. Item, Gilberti Porretae, Episcopi Pictauiensis in IIII.Lib.de Trinitate commentarij, antè nunquam aediti. Praeter reliquos doctiß.uiros, Henricvs Loritvs Glareanvs, Aritmeticam et Musicam demonstrationibus et figuris auctiorem redditam, suo pristino nitori restituit: ... Et Martianvs Rota, opus de tota disserendi ratione, hoc est, organum, ... illustrauit. Et huius autoris uitam, ... descripsit. .(1570)
https://books.google.com/books?id=1h3p0s_7abMC&pg=PT567
search - tres funt unum - five hits
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Check Neckham and Gilbert of Poitiers on Boethius
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Horne includes in list of omitters
https://books.google.com/books?id=pMUVAAAAYAAJ&pg=PA370
Davidson too
http://books.google.com/books?id=ygEVAAAAYAAJ&pg=PA406
Faith, Reason and Theology: Questions I-IV of His Commentary on the De Trinitate of Boethius (1987)
https://books.google.com/books?id=RucJa3w2QrMC&pg=PR3