Usage: 2 forms in 2 verses
Form Translated As     In Verse(s)
τριετήσ a three years LXX.Isa.15.5
τριέτουσ three years LXX.2Chr.31.16

Liddell-Scott-Jones

τριέτης (trietēs) of three years G:A

τριέτης-ης, ες,

or τριετής, ές, (ἔτος) of or for three years, τριέτεα χρόνον [Refs 5th c.BC+]; πλέον τ. ἐγένευ φίλος [Refs 3rd c.BC+]; τ. προθεσμία [Refs 5th c.BC+] as adverb, for three years, [Refs 8th c.BC+]

__2 three years old, ἵππος [Refs 4th c.BC+] the age of three years, ἀπὸ τριέτους μέχρι τῶν ἓξ ἐτῶν [Refs 5th c.BC+]:—feminine τρῐετ-έτις, [Refs]
__II recurring every three years, κῶμος [Refs 5th c.AD+]