Forcing Unity to delete read-messages after three days seems a bit drastic to me. Please read this first - Best Practices for Managing Subscriber Mailbox Size
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/products_maintenance_guide_chapter09186a0080229b48.html#wp1061992
If you still want to do it, use the Message Store Manager utility. These are the scripts that you are interested in:
Move Messages from Inbox to Unity Dumpster
Moves messages from each subscriber’s Inbox to a special folder called “Unity Dumpster”. The Unity Dumpster folder will be created by the MSM for each subscriber if it does not already exist as a sub-folder of each subscriber’s Inbox. It can be used as a temporary holding tank for messages that are no longer important enough to keep in the Inbox, but not yet considered so useless as to merit deleting.
Move Messages from Inbox to Deleted Items Folder
Moves messages from the Inbox to the Deleted Items folder for each subscriber in the Agent.
For exchange, you could move the transactional logs to a drive with more space. An exchange backup clears the transactional logs. An alternate is to enable Circular Logging
Moving Exchange Message Stores and Transaction Logs to an Alternate Drive
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/products_tech_note09186a00800c6f5f.shtml
Circular Logging in Exchange
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/products_maintenance_guide_chapter09186a00801c1ebc.html#1067934
