You can tell what declension a Latin noun is by looking at the noun's genitive singular form.
The third declension is one of the worst, and it might be simplest to memorize each gender as you learn the dictionary form of the word.
Third declension.
The Latin noun Homo, hominis is in the third declension.
The Latin word cīvitās in the nominative case, cīvitātis in the genitive, is a third declension feminine noun.
Each latin noun has five forms referred to as declension.
"Beef" in Latin is the 1st-declension feminine noun bubula.
Luna means "moon" in Latin. It is a first declension feminine noun.
Argicola, agricolae, feminine, a first declension noun.
'officina, officinae'. It is a first declension noun.
Rebellionis - masculine noun, lll declension
'Forum' (a neuter, 2nd declension noun) is used in Latin for "marketplace."
arbor, arboris. It is a 3rd declension noun.
"Freedom" in Latin is the feminine third-declension noun libertas (genitive libertatis).