Bhagavad Gita, Chapter 17, Verse 26

The word Sat is used in the sense of reality and of goodness; and so also, Pârtha, the word Sat is used in the sense of an auspicious act.

The third word gets defined here plainly, real eternal existence, not just another ceremonial term.

Krishna names Sat directly as eternal and good, giving the final syllable of this trio real philosophical weight. Meaning, not just sound, is what's being explained.