The affix tīya comes in the sense of 'completer thereof' after the word tri and there is samprasāraṇa (Vocalisation) of the stem.,
The substitution of a vowel for a semi-vowel is samprasarana (I. I. 45). Thus त्रि + तीय = तृ + इ + तीय = तृतीयः (6.1.108, the इ of त्रि assumes the form of the prior letter ऋ which is substituted for र्). In तृतीयः the ऋ is not lengthened by 6.4.2, because that rule applies to the lengthening of the अण् letters only, this word being read into 6.4.2, by anuvritti from (VI. 3. III). The pratyahara अण् is formed by the first ण्, and means the letters अ, इ and उ ॥,