I suppose this is possible if you want the envelope to stay open for several months. Just create the envelope set 3 recipients as "Needs to Sign" with the exact same email address and name for each Recipient, attach the documents, drop the appropriate color coded tags and I would suggest changing the Reminders and Expirations and Send. The envelope doesn't complete until all three signatures across all three documents but at least you can track progress from one signature to the next. Once all three signatures are in place, the envelope is completed and Signers and Sender gets a copy.

The alternative of course is send one doc for Signing, then perhaps use the Forward (or copy) option when you need the next document to be signed and add the second document. This creates a new envelope containing the existing signed doc plus the new doc.....repeat for document 3.