You can watch the round three (Dodger Stadium) on CBS sunday at 12:00 eastern time.
You can see all the races this year on either SPEED or CBS. Some are "live" and some will be shown the next day. You can get a TV schedule at www.motocrossaction.com. Another good source of up to the minute race info would be Steve Cox's reports at Motocross.com / GrindTV.com. Cox covers every single Supercross Race and outdoor National on the circut and can be relied on for the most accurate and inside info.