Sorry for the delay in response.
As per your statement above, do you mean to say once the document is signed, it automatically sent to a third email address without any notification to others?
If not it would be helpful if you can elaborate a bit what exactly you are trying.
If you just wish to send a copy of the signed document to some other email address, there is a setting where you can add the alternate email address.
However, the setting is not provided in the individual account for Adobe Sign.
It should be the business level account.
You may try this workaround when sending the document for signature, you can add that email address as an approver after adding the signer.
Once the document is signed, it will be forwarded to the other email address. Check the screenshot below:
Hope that helps.
Let us know if you have any question.