Multiple signatures in a document library workflow

Ok Here is the situation:

This is the flow of work.

A document gets put into a document library. That’s when the workflow should start by sending an email to person 1. Person 1 signs the document (By using a third party signature product), SAVES it, whereby a second emaiil is sent out alerting person 2 there is a document waiting for them to reviewand or sign. Person 2 signs it an email gets sent to person 3, etc…..All the while this document is sitting in the document library being changed by being signed and saved. My problem is, once it is saved by every person, how do I let the workflow know to send it to the nexxt person? Do I use versioning? How do I kick this off? Does anyone have any ideas?

Thanks in Advance..

Pat