According to dictionary.com, the origin of the term triune was between:
1595–1605; tri- + -une < Latin ūnus one
What are some of the earliest uses of the English triune, as well as it's Latin and Greek (if such exist) equivalents?
The OED gives this as the earliest English usage of the term "triune":
1605 T. Tymme tr. J. Du Chesne Pract. Chymicall & Hermeticall Physicke ii. ii. 108
It hath pleased the omnipotent Creator to..showe himself a Unitrine or Triune.
According to the Index Thomisticus, triunus (or any of its forms) is not found in the works of St. Thomas Aquinas, so the Latin term triunus is probably more recent than the 13th century.