Katha-1-1-18-triṇāciketas

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

He now concludes the fruits, of the knowledge and the performance of sacrifice and with them, the present topic. The three-fold Nachikêtas who properly understands what was said about the bricks, their numbers and mode and who realizing the fire as the âtman completes the sacrifice called Nachikêta, shakes off even before death, the meshes of Death consisting in vice, ignorance, desire and hatred, etc., and devoid of all grief rejoices in heaven, i.e., by realizing his Self as the virât.

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