The driver all the way at the back who first made contact is most likely at fault for the crash. That driver would be held liable for damages and injuries inflicted on the two drivers in front.
Evidence and witnesses are key to determining fault in auto accidents. If the fault is not clear at the scene of the accident, there may be traffic cameras that recorded the accident. By reviewing the footage, a driver's fault may be able to be assigned sooner than later this way. Photographic evidence can help, too.