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SB 11.17.12

tretā-mukhe mahā-bhāga
 prāṇān me hṛdayāt trayī
vidyā prādurabhūt tasyā
 aham āsaṁ tri-vṛn makhaḥ
tretā-mukhe — at the beginning of Tretā-yuga; mahā-bhāga — O greatly fortunate one; prāṇāt — from the abode of prāṇa, or the life air; me — My; hṛdayāt — from the heart; trayī — the threefold; vidyā — Vedic knowledge; prādurabhūt — appeared; tasyāḥ — from that knowledge; aham — I; āsam — appeared; tri-vṛt — in three divisions; makhaḥ — sacrifice.
O greatly fortunate one, at the beginning of Tretā-yuga Vedic knowledge appeared from My heart, which is the abode of the air of life, in three divisions — as Ṛg, Sāma and Yajur. Then from that knowledge I appeared as threefold sacrifice.

In Tretā-yuga, the bull of religion loses one leg, and only seventy-five percent of religious principles are manifested, represented by the three principal Vedas — Ṛg, Sāma and Yajur. The Lord appears in the process of threefold Vedic sacrifice. The three divisions are understood as follows. The hotā priest offers oblations into the fire and chants the Ṛg Veda; the udgātā priest chants the Sāma Veda; and the adhvaryu priest, who arranges the sacrificial ground, altar, etc., chants the Yajur Veda. In Tretā-yuga such sacrifice is the authorized process for spiritual perfection. The word prāṇāt in this verse refers to the universal form of the Personality of Godhead. This form is further described in the following verses.