Does placing P & C on a email in the text provide any legal rights to it being P&C if the person has distributed the document to several people and slanders a third party that is not copied on the email. can I forward it to the tird party?
Maybe. Without any context this is hard to answer. The backstory and context may make the difference as to whether the sender suffers damage because you disclosed confidential information. There are many parts to what appears to be a simple question and not something that can readily be answered in a forum.
And defamation is still defamation regardless of any disclaimer about confidentiality. If the 3rd party were to somehow get their hands on the material they can use it regardless of any P&C disclaimer.