Is expiation (kaffarah) obligatory upon the driver whose car collided with another car that suddenly cut in front of him, resulting in the death of three people, despite his attempt to avoid the collision?

If it is not possible to avoid the collision or reduce the speed, then there is nothing [no liability] upon the driver. However, if he was able to do so but neglected it, then he is obligated to pay the blood money (diyya) and offer expiation (kaffarah) for each person killed in the accident.