Hi,
So I understand what you are trying to do but I think the option you are looking for is actually a SharePoint option rather than a Nintex one. You are trying to provide a Reminder 3 month before a due date is reached. You would do this using the Information Management policies on the Library. Essentially go to Library Settings > Information management policies.
You are going to set up an Expiration policy in the guise of a retention policy for the documents in the library. You can create a column called Expiry date. You can then set up an expiration policy that kicks off 3 months before the date (Or you can make the column reminder data and kick off the expiration on that reminder date).
In terms of the rules you set up you can then kick off a workflow to remind you to update the document.
The benefit of this approach is you dont have to have long running workflows waiting for a document to expire / or to write a daily routine that might need to iterate through many documents.
https://support.office.com/en-us/article/Create-and-apply-information-management-policies-eb501fe9-2ef6-4150-945a-65a6451ee9e9?ui=en-US&rs=en-US&ad=US
Hope this helps otherwise drop me a message and I can help you set it up
Craig