1. They belong to 1st , 2nd , and 3rd declension
Adjectives II
They belong to 1st , 2nd , and 3rd declension
They agree with a noun in gender, number,
and case

2. 1st and 2nd declension adjectives agree with their nouns also in declension.
They are listed in the dictionary with 3 endings:
M F N
laetus laeta laetum
laetus puer laeta puella laetum donum
happy boy happy girl happy gift
Of course this words are in the NOMINATIVE case

3. 3rd declension adjectives do not agree with their nouns in declension
They are listed in a dictionary as:
M F N
celer celeris celere
swift swift swift
celer iuvenis celeris femina celere prandium
swift youngmen swift woman swift meal

4. 2 endings in Nominative: one form for masculine and feminine, and one form for neuter: M F N
brevis, breve
M F N
brevis stilus brevis vita breve bellum
short pen short life short war

5. 1 ending for all tree genders:
M F N
Nom./Voc. felix felix felix
Gen felicis felicis felicis
Dat felici felici felici
Acc felicem felicem felix
Abl felici felici felici

6. felix homo felix puella felix carmen happy man happy girl happy song