त्रेस्त्रयः

Operational rule vidhi

The word त्रि gets the आदेश 'त्रय' when followed by the प्रत्यय 'आम्'.

Word by word (padaccheda)

Target त्रेः ṣaṣṭhī · ekavacanaSubstitute / term त्रयः prathamā · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

Dotted words are technical terms. Hover or tap one to see its card.

Full reading

त्रेः त्रयः

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

As a rewrite rule (sketch from case endings)

त्रेः → त्रयः

Replace treḥ with trayaḥ. Inherited words (anuvṛtti) may add further conditions.

Commentary

त्रय is substituted for त्रि before the Genitive 6th-Case plural affix.
As त्रयाणाम् ॥ त्रीणाम् however appears in the Veda: as त्रीणामपि समुद्राणाम् ॥

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.