Allen and Greenough's New Latin Grammar, section 114

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THIRD DECLENSION (CONSONANT AND i-STEMS).

114.

Adjectives of the Third Declension are thus classified: -

1. Adjectives of Three Terminations in the nominative singular, - one for each gender: as, ácer, ácris, ácre.

2. Adjectives of Two Terminations, - masculine and feminine the same: as, levis (M., F.), leve (N.).

3. Adjectives of One Termination, - the same for all three genders: as, atróx.

a. Adjectives of two and three terminations are true i-stems and hence retain in the ablative singular -í, in the neuter plural -ia, in the genitive plural -ium, and in the accusative plural regularly -ís (see §§ 73 and 74).[1][But the forms of some are doubtful.]