Car Accident, who's fault is it?

It was a green light, and the 1st car was at a dead stop for a while, and the I guess the 2nd car noticed the 1st car and suddenly slammed on the brakes, and then I (3rd car) slammed on the brakes and skid and hit the 2nd car, the 2nd car and me was traveling 20mph while the 1st car was at a dead stop on a green light; so who's fault is it?