For actions that are supposed to occur at specific intervals (rather than when a ticket is updated as with triggers) you'll want to set up an action schedule. To create an action schedule, first you'll want to create a report that hits all the criteria that you want found in the incidents. (Reports can be created by going to Incidents->Reports & Queues->New Report) For your example, I am guessing you will want to make a report of all incidents that are due to be resolved in three days, something like the below.
Once you create and save the report, you can create an action schedule with the action you would like to occur. For information on making an action schedule, please see the below help article.