Katha-1-1-17-triṇāciketastribhiretya

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

Again he praises Karma; the three-fold Nachikêtas, i.e., he by whom the Nachikêta fire has been kindled thrice; or, he who knows, studies and performs in, the Nachikêta 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 ‘ágamas or to ‘the vêdas,’ ‘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 vêdas, 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. Dêvam, so called because shining, i.e., having the qualities of knowledge, etç. Idyam, worthy of praise. Knowing such fire, from the Sâstras and having realised him as his own âtman, one attains this absolute renunciation which is realized in his intellect. The meaning is that one attains the place of the virât, by the continued practice of Upâsana and Karma.

Links
Various resources on the Kathopanishad
Upanishad Series – Katopanishad Commentary by Swami Chinmayananda