He brooded over these three deities, and from them thus brooded on he squeezed out the essences, the Rik verses from Agni, the Yagus verses from Vâyu, the Sâman verses from Âditya.

In plain words: Then Prajapati thought deeply about fire, air and sun and drew out their essence. He got the three kinds of Vedic verses: Rig verses from fire, Yajur verses from air, and Sama verses from the sun.