USC coach Pete Carroll has had three cars parked in his driveway for some time now. One has been a three-week loaner, the other two have not been out for a test drive. Mark Sanchez, Mitch Mustain and Aaron Corp will be on full display when the Trojans commence spring practice Tuesday. With USC likely to open the season ranked in the top 5, its quarterback competition could be a national storyline in college football this spring.