Third declension

The third declension is a category of nouns in Latin and Greek with broadly similar case formation — diverse stems, but similar endings. Sanskrit also has a corresponding class (although not commonly termed as third), in which the so-called basic case endings are applied very regularly.

In contrast with the first- and second-declension endings, those of the third declension lack a theme vowel (a or o/u in the first and second declensions) and so are called athematic.

One distinguishing feature of third-declension nouns is a genitive singular ending of a short vowel and s: Latin rēg-is "of a king" Greek χειρ-ός (cheir-ós) "of a hand", and Sanskrit bhagavat-as "of the blessed (one)". Another is a dative singular ending of i (short i in Greek, long ī in Latin): rēg-ī "for a king"; χειρ-ί (cheir-í) "for, with the hand". This corresponds to an -e ending in Sanskrit, which might have been a contracted ai or lengthened i: bhagavat-e "for the blessed (one)"

Many third-declension nouns, unlike first- or second-declension nouns, show different stems depending on case and number — usually one stem for the nominative singular, and another for the rest of the cases, though some Greek nouns have three stems. Greek stems are often formed by ablaut: Latin "person" and homin-ēs "people"; Greek (patēr’) "father", πατρ-ός (patr-ós) "of a father", and πατέρ-ες (patér-es), "fathers". In Sanskrit the situation is similar to that in Greek, but the strongest stem is used somewhat more.

...
Wikipedia