You need to win all 9 Time Attack events (one from each District) in the Shatter Bay Career. In Time Attack you start with a timer and you have to reach the end of the track before time runs out.
The 3 star time for most of the events are quite difficult and will take you more than several attempts until you learn the track well enough to master it. Some of these events include cop cars that will try to slow you down, others include ramps and jumps with the blue RR objects that freeze time for a few seconds if you collect them. Collecting these objects is vital as it will make a huge difference in gaining a 2 star and a 3 star rating.
The best car for each car class is the Hurricana CX (Super class), Dorean LE (Race class) & the Motega XR (Street class) but you can still experiment to find the car that best suits you for each class.