From what I have read, most third-declension Latin adjectives other than comparatives take the i-stem endings -ī, ium and -ia in the ablative singular, genitive plural and neuter nominative/accusative plural, respectively.
Allen and Greenough §114-119 discuss the general patterns, and Allen and Greenough §121 discusses -e vs. -ī in the ablative singular, -ium vs. -um in the genitive plural, and a few other variations like -īs vs. -ēs in the masculine/feminine accusative plural and feminine nominative singular forms in -a, like clienta and hospita.
Adjectives that take -a in the neuter plural nominative/accusative appear to be even less common than adjectives that take -um or -e, though. It seems that many third-declension adjectives of one ending lack attested neuter plural forms at all (discussed to some extent in Allen and Greenough §122).
Which third-declension adjectives have attested plural forms in -a?