Ternis horárum términis

I am trying to understand the meaning of this hymn, found in the Roman Catholic Afternoon prayer (Nona) in the Roman Breviary (Liturgy of the hours)

"Ternis horárum términis volútis, dante Dómino, trinum perfécte et únicum ipsum devóti psállimus".

I have found the following French translation, that goes: "Déjà la grâce du Seigneur - Trois fois nous a donné trois heures ;- Avec amour nous chantons Dieu,-
Trinité, parfaite Unité".

I know that "ternis" means "three times" but then why...

Ternis horárum términis