Om Tat Sat, this is remembered as the threefold designation of Brahman. By that, the brahmins (brāhmaṇāḥ), the Vedas, and the sacrifices (yajñāḥ) were ordained of old.
OM TAT SAT: “That Is the Real”— this is the triple symbol of the infinite spirit that gave a primordial sanctity to priests, sacred lore, and sacrifice.
(23) OM—THAT—IT IS: This has been handed down, a three-fold pointer to Brahman: by this were allotted their proper place of old Brahmans, Veda, and sacrifice.
17.23 'Om-tat-sat' ['Om, That, Existence': 'Om iti brahma, Om is Brahman' (Tai. 1.8.1); 'Tattvamasi, Thou art That' (Ch. 6.8.7); and 'Sadeva somya idamagra asit, This was Existence alone in the beginning, O amiable one' (Ch. 6.2.1)-in these texts Brahman is indicated by the words Om, tat, sat.]-this is considered to be the threefold designation of Brahman. The Brāhmaṇas and Vedas and the sacrifices were ordained by that in the days of yore.
17.23 Om, Tat, Sat --- thus Brahman is denoted by this threefold expression. Associated with these, Brāhmaṇas, the Vedas and sacrifices were ordained in the past.
17.23 "Om Tat Sat": This has been declared to be the triple designation of Brahman. By that were created formerly, the Brāhmaṇas, the Vedas and the sacrifices.