On the Feast of the Most Holy Trinity, in the Divine Office of Prime, the Athanasian Creed is said. And it leaves no doubt as to the unity of the Holy Trinity! It has been used by Christian churches since the sixth century, and is the first creed in which the equality of the three persons of the Trinity is explicitly stated. There’s an interesting article at Wikipedia.
R. As it was in the beginning, is now, * and ever shall be, world without end. Amen.
Ant. Glory be to thee, O equal Trinity * one Deity, from before all ages, so now and for evermore.