Case 3 - Dead Rising
Case 3 is just a cutscene. Be in the Security Room at 11:00AM to complete it. You'll then have until 3:00PM, when Case 4 begins. Consider completing some scoops between now and then. Just keep in mind that you have to be in the Security Room at 4:00PM; don't stray too far from the Security Room when that time nears.
If you didn't encounter Pamela and Heather Tompkins on your last trip through Paradise Plaza, head there now and they should spawn.