Why not tertia cliens in aedificio est ?
"Cliens" is masculine singular.
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.
Doesn't the number normally come before the subject? Here the subject is first (client) and then the number (third).