I have never sung any of the Propers for Holy Trinity before so I was surprised to find the Canticle Benedicta es as an alternative option to the Gradual.
Holy Trinity is a (relatively) late addition to the calendar, though the votive Mass was in use much earlier. Almost all the melodies are taken from earlier pieces (e.g., Introit from 1 Lent, gradual & offertory from SS Peter & Paul) and adapted as needed for the texts of the formulary.
In the /GR /for the EF you will find this hymn sung as a response to the fifth reading on Ember Saturday in Advent (day prior to 4th Sunday of Advent). The four Ember Saturdays had many readings and they were typically ordination days, with ordination to the various minor and major orders coming after the various readings, each of which had a proper gradual.
It comes, of course, from the canticle of the three young men in the fiery furnace (Dn 3:56ff). Since the very early days of the Church, it has been a favourite text of Christian writers and poets who saw it as a prophecy of the doctrine of the Holy Trinity. (Sung each Sunday, Feast, and Solemnity at Lauds, it is the only canticle in the Divine Office that retains its own unique doxology instead of the Gloria Patri.)
When Solesmes was commissioned to assemble the new Graduale for the OF, according to the principles and directions in the Ordo Missae, the Ember Days were no more. So this venerable hymn was given as an alternative to the gradual of Trinity, as both come from the same biblical text.
Thanks to Stan Metheny for enlightening me as per usual