The singular nominative of the third declension can end in –s :

Cūstōs servat pīrātam = The jailer is guarding the pirate. (cūstōs = jailer. Servāre = to guard)

Normally third declension masculine nouns in the singular nominative end in –s, but that’s not to say that all third declension nouns that end in –s of the singular nominative are masculine :

Fēlēs volvit = the cat is tumbling/rolling. (fēlēs = cat. Volvere = to roll, tumble)

Even though fēlēs is a feminine noun, it ends in –s of the singular nominative.

The third declension is notorious for the variety of endings that fall in the third declension category. For instance, you might see some third declension singular nominative nouns that end in –x :

Cervīx movet = the neck is moving (movēre = to move. cervīx = neck)

But just because a third declension noun ends in –x of the singular nominative doesn’t mean that the noun is feminine :

Rēx dēdit  = the king is surrendering. (dēdere = to give up, to surrender.)

There are third declension feminine singular nouns in the nominative that end in –r :

Māter portat cibum = the mother is carrying the food. (māter = mother)

But third declension masculine singular nouns in the nominative can also end in –r :

Pater portat aquam = the father is carrying the water. (pater = father)

There are some third declension neuter nouns that end in –n :

Flūmen crēsit = the river is growing, increasing. (flūmen = river. crēscere = to grow, to increase.)

But not all third declension neuter nouns in the nominative singular ends in –n. Some end in –s :

Opus crēsit = the work is increasing, growing. (opus = work)

There are also third declension nouns that end in –l :

Animal frangit casam = the animal is breaking/shattering the house. (Animal = animal. frangere = to break, to shatter)