Third declension nouns can get messy. There are lots of different conjugations. For today, just the nouns that add -s to the stem of the noun to form the nominative singular will be listed. These nouns will be either masculine or feminine.

N : –s

G : –is

D : –ī

Acc : –em

Abl : –e

The singular endings for the Latin word for soldier:

N : mīles

G : mīlitis

D : mīlitī

Acc : mīlitem

Abl : mīlite

The endings for the Latin word for king:

N : rēx

G : rēgis

D : rēgī

Acc : rēgem

Abl : rēge