त्रेस्त्रयः

Operational rule vidhi

The words त्रि gets an आकारादेश in presence of a सङ्ख्याशब्द, provided that there is no बहुव्रीहि समास and the word अशीति is not present.

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.

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

For त्रि is substituted त्रयस् when another numeral follows but not in a बहुव्रीहि समास or before आशीतिः।
As, त्रयोदश, त्रयोविंशतिः, त्रयस्त्रिंशत् ॥ But not in त्रैमातुरः (the second word not being a Numeral), nor in त्रिदशाः which is a Bahuvrihi, 2.2.25 त्र्यशीति ॥ This substitution takes place upto hundred : not here, त्रिशतम्, त्रिसहस्रम् ॥

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