Upon whom is the expiation for fasting due in the car accident that was caused by a car hitting my husband from behind, leading my husband to hit the car in front, and a person in it died due to an iron bar entering his abdomen because the passengers were thrown from the back to the front? Is it on the owner of the first car, or my husband, or the passengers in the third car?

It appears that the indemnity (daman) and expiation (kaffarah) are incumbent upon the 'aqilah (male relatives on the father's side) of the person who caused the accident. As for the husband, he is not liable for anything if he was unable to control his car after the collision, because in that case, he is like an instrument, and the indemnity is upon the one who used him as an instrument. However, if he was able to control it and avoid the collision, then the indemnity and expiation are incumbent upon his 'aqilah.