This might work well:

Create an Automated Action that uses a timed condition based on the creation date of the Signature Request:

The Email Template (1) is what will be sent to the Customer. 

You would setup one Automated Action for each interval - 3, 6, 9, 12 days - and then one final Automated Action for the 15 day interval - that one telling the Customer that the Contract would not be continued.