def exists(self, folderName, serviceName=None, serviceType=None):
        """
        This operation allows you to check whether a folder or a service
        exists. To test if a folder exists, supply only a folderName. To
        test if a service exists in a root folder, supply both serviceName
        and serviceType with folderName=None. To test if a service exists
        in a folder, supply all three parameters.

        Inputs:
           folderName - a folder name
           serviceName - a service name
           serviceType - a service type. Allowed values:
                "GPSERVER", "GLOBESERVER", "MAPSERVER",
                "GEOMETRYSERVER", "IMAGESERVER", "SEARCHSERVER",
                "GEODATASERVER", "GEOCODESERVER"
        """

        url = self._url + "/exists"
        params = {
            "f" : "json",
            "folderName" : folderName,
            "serviceName" : serviceName,
            "type" : serviceType
        }
        return self._post(url=url,
                             param_dict=params,
                             securityHandler=self._securityHandler,
                             proxy_url=self._proxy_url,
                             proxy_port=self._proxy_port)