def convert(self, alpha3, country=None, script=None):
raise NotImplementedError<EOL>
Convert an alpha3 language code with an alpha2 country code and a script code
        into a custom code

        :param string alpha3: ISO-639-3 language code
        :param country: ISO-3166 country code, if any
        :type country: string or None
        :param script: ISO-15924 script code, if any
        :type script: string or None
        :return: the corresponding custom code
        :rtype: string
        :raise: :class:`~babelfish.exceptions.LanguageConvertError`