"The third client is in the building."

Translation: Cliens tertius in aedificio est.

[Thomas50510]

Why not tertia cliens in aedificio est ?

[mahdaeng]

"Cliens" is masculine singular.

[CannedMan]

Wiktionary claims that cliēns, -entis is both masculine and feminine, but that is not supported by the listed sources. There is indeed a feminine form of the word, but it is of the first declention: clienta, -æ.

[Edward42922]

Doesn't the number normally come before the subject? Here the subject is first (client) and then the number (third).