स एतास्तिस्रो देवता अभ्यतपत्तासां तप्यमानानां रसान्प्रावृहदग्नेरृचो वायोर्यजूंषि सामान्यादित्यात् ॥ ४.१७.२ ॥
In plain words
He brooded upon these three deities; and from them, as he brooded, he drew out their essences: the verses of the Ṛc from fire, the formulas of the Yajus from the wind, the chants of the Sāman from the sun.
The commentators
Chāndogyopaniṣadbhāṣyam· bhāṣya
पुनरप्येवमेवाग्न्याद्याः स एतास्तिस्रो देवता उद्दिश्य अभ्यतपत् । ततोऽपि सारं रसं त्रयीविद्यां जग्राह ॥
In plain words
In this exact same manner, focusing once more upon Agni and the other deities, he subjected them to ascetic heat. From them as well he gathered the ultimate essence—the threefold Vedic wisdom.
Close to the Sanskrit
In this very same way once more, targeting Agni and the other deities, he brooded upon these three deities. From them too he gathered the essence, the triple knowledge.