Third declension adjectives of three terminations have three different forms for their nominative singular — one for each gender.

Table 6-4 Third Declension Adjectives of Three Terminations: ā cer,ā cris, ā cre (“sharp, fierce”)

In the dictionary listing ā cer,ā cris,ā cre (“sharp, fierce”), the first form, ā cer, is the masculine nominative singular, the second is the feminine, and the last one is neuter. Based on this adjective classification system, you could say that all first/second declension adjectives are adjectives of three terminations, since they all have three nominative singular forms (–us, –a, –um), a different one for the masculine, feminine, and neuter.