Text 17 

shraddhaya paraya taptam
tapas tat tri-vidham naraih
aphalakanksibhir yuktaih
sattvikam paricakshate

Translation

This threefold austerity, performed with transcendental faith by men not expecting material benefits but engaged only for the sake of the Supreme, is called austerity in goodness.

Commentary by Srila Prabhupada

No commentary by Srila Prabhupada.

Commentary by Sri Visvanatha Cakravarti Thakur

The three types of austerity are those of body, words, and mind (tat tri vidham).