Karnatakabhashabhushana 6.32.2

6.32.2

trisaṅkhyāvācinaḥ śabdasya kapavargayōḥ parayōḥ tāvāgamau hrasvapūrvau bahuḷaṁ bhavataḥ.

When [a consonant of] the ka-class or pa-class follows, those [consonants] variously appear as augments preceded by a short [vowel] for a word denoting the number three.