Srimad Bhagavatam Canto 11, Chapter 17, Text 12

SB 11.17.12

treta-mukhe maha-bhaga
 pranan me hrdayat trayi
vidya pradurabhut tasya
 aham asam tri-vrn makhah
O greatly fortunate one, at the beginning of Treta-yuga Vedic knowledge appeared from My heart, which is the abode of the air of life, in three divisions — as Rg, Sama and Yajur. Then from that knowledge I appeared as threefold sacrifice.
In Treta-yuga, the bull of religion loses one leg, and only seventy-five percent of religious principles are manifested, represented by the three principal Vedas — Rg, Sama and Yajur. The Lord appears in the process of threefold Vedic sacrifice. The three divisions are understood as follows. The hota priest offers oblations into the fire and chants the Rg Veda; the udgata priest chants the Sama Veda; and the adhvaryu priest, who arranges the sacrificial ground, altar, etc., chants the Yajur Veda. In Treta-yuga such sacrifice is the authorized process for spiritual perfection. The word pranat in this verse refers to the universal form of the Personality of Godhead. This form is further described in the following verses.
Srimad Bhagavatam Canto 11, Chapter 17, Text 11
Srimad Bhagavatam Canto 11, Chapter 17, Text 13