I have been in multiple car pile-ups twice. The first time I was the car in the middle of three cars, and the second time I was in the middle of about a dozen cars. In both cases, the driver in the very rear was the responsible party for all of the cars damaged. If the police report indicates that the car that hit you was indeed hit by another car that took off, then I think the car that hit you isn't responsible. If the report makes no mention at all of a 3rd unknown vehicle, then the car that struck you would be responsible. This sounds like one for the insurance companies to battle out. In both cases when I was hit, I gave all of the information to my insurance agent who helped me through the process. Call your agent, even if you don't file a claim against your own insurance, and ask his advice. Good luck!
You would need to provide more details to establish who was At Fault, but if the hit and run vehicle caused the accident and this is verifies by all the three drivers in the pile up it would most likely fall to be the responsibility of the hit and run driver and in turn the motor insurer bureau. Pile ups are common in motorway collisions - see the related link entitled "motorway accident claim" to see how to know who is at fault in a UK motorway crash.