You definitely want to read the admin documentation on scheduler.
First, you should note that scheduler runs in UTC, not your local time zone.
Second, your scheduler jobs can only run every 10 minutes (which should be more than enough in this case).
If you want your reminders to be sent at around three days after the last contact from the agent, you could set up something like this:
Make sure that you filter the state of the ticket. If you forget to exclude closed tickets things could get quite messy
Then add the email notification under ‘execute changes on objects’. EDIT: Also, don’t forget to add a state change (as @MrGeneration fortunately added below):
EDIT: Please read the other comments below as well
If that solves your case please mark this post as a solution.
If you’ve got any more questions don’t hesitate to post a follow-up.