Signing a message to make sure that it will not be tampered with when forwarded, without trusting the receivers?
I see this as a mixture of easy and impossible.  A standard digital signature will work to verify the contents, as long as the document+signature is sent to person 1 (or whoever wants to check the document)

The impossible part comes if you only want person 1 to only have the signature and not the document.  Even if person 1 keeps a copy of every signature, Person 2 can send a tampered document with the original signature.  Person 3 would know the document has been tampered, but if 3 wants to be part of the deception they can just send the original signature to person 1.