def get_representative_cases(self):
        """
        >>> armr = OldNorseNoun("armr", decl_utils.Gender.masculine)
        >>> armr.set_representative_cases("armr", "arms", "armar")
        >>> armr.get_representative_cases()
        ('armr', 'arms', 'armar')

        :return: nominative singular, genetive singular, nominative plural
        """
        return (self.get_declined(decl_utils.Case.nominative, decl_utils.Number.singular),
                self.get_declined(decl_utils.Case.genitive, decl_utils.Number.singular),
                self.get_declined(decl_utils.Case.nominative, decl_utils.Number.plural))