त्रिप्रभृतिषु शाकटायनस्य

Operational rule vidhi

According to शाकटायन, when three or more letters are joined using a संयोग, the द्वित्वम् does not happen.

Word by word (padaccheda)

Right context त्रि- प्रभृतिषु saptamī · bahuvacanaTarget शाकटायनस्य ṣaṣṭhī · ekavacana

Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

Full reading

त्रि-प्रभृतिषु शाकटायनस्य

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

Commentary

According to the option of शाकटायन , the doubling does not take place when the conjunct consonants are three or more in number.
A इन्द्रः, चन्द्रः, उष्ट्रः, राष्ट्रम्, भ्राष्ट्रम् ॥

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