Unless otherwise stated for a specific driver the option for url () should use a SPACE to separate multiple entries.
http(url("http://your-elasticsearch-server/_bulk" "http://your-second-ingest-node/_bulk" "http://your-third-ingest-node/_bulk")
For more information please see the Syslog-NG PE Administrator Guide here about this specific entry.
It is recommended to check your specific destination documentation to make sure there are no differences or special requirements. You can find the Admin Guide here.