त्रिणाचिकेतस्त्रिभिरेत्य सन्धिं त्रिकर्मकृत्तरति जन्ममृत्यू ।
ब्रह्मजज्ञं देवमीड्यं विदित्वा निचाय्येमाँ शान्तिमत्यन्तमेति ॥ १७ ॥

triṇāciketastribhiretya sandhiṃ trikarmakṛttarati janmamṛtyū ।
brahmajajñaṃ devamīḍyaṃ viditvā nicāyyemām̐ śāntimatyantameti ॥ 17 ॥

17. The three-fold Nachiketas, being united with the three doing, the three-fold Karma, crosses birth and death, knowing the adorable, the bright, the omniscient fire born of Brahman and realising him, attains thorough peace.

Shankara’s Commentary:

Again he praises Karma; the three-fold Nachiketas, i.e., he by whom the Nachiketa fire has been kindled thrice; or, he who knows, studies and performs in, the Nachiketa fire. United with the three, i.e., who united with his mother, father and preceptor, i.e., having duly received instruction from them; for, that such instruction is a source of authority, is inferred from other Srpitis, such as ‘he who has a mother’ and ‘he who has a father,’ etc. or, the three may refer to ‘direct perception,’ ‘inference’ and ‘agamas or to ‘the vedas,’ ‘the Smritis’ and ‘good men’; for, knowledge of virtue from these sources is an obvious fact. Doing the threefold karma, i.e., performing sacrifices, reciting the vedas, and making gifts. Whoso does these, crosses or travels beyond birth and death; again Brahmajagnam: Brahmaja means born of Brahma, i.e., Hiranyagarbha; he who is born of Brahma and is omniscient is Brahmajagnah. Devam, so called because shining, i.e., having the qualities of knowledge, etç. Idyam, worthy of praise. Knowing such fire, from the Sastras and having realised him as his own atman, one attains this absolute renunciation which is realized in his intellect. The meaning is that one attains the place of the virat, by the continued practice of Upasana and Karma.