All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is the person who caused the accident who is obliged to expiate. If the person who was driving the care in which there were three passengers was the reason why the trailer hit his car, then it is him who is obliged to expiate and pay the Diyah (blood money), and if it was the trailer driver who caused the accident without the car driver being at fault, then it is the trailer driver who is obliged to expiate and pay the Diyah.
In this accident which you mentioned in the question, the person who caused the accident is obliged to perform three expiations and pay the Diyah for three persons; one expiation and one Diyah on every dead person.
For more benefit on the Diyah and expiation, please refer to Fatwa 91306.
Allaah Knows best.