'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`'

def convert(self, alpha3, country=None, script=None):

    raise NotImplementedError
