Translation:Suntne tres sorores?
No feminine three? Always tres?
The lexical form of three is "tres, tres, tria". It is like a 3rd declension adjective; same in the masc and fem, different in the plural. Check this out https://en.wiktionary.org/wiki/tres#Latin
tres is both masculine and feminine in the nominative, accusative, and vocative. tria would be used for the neuter in those cases.