I guess it depends where u hit the middle car, in most cases the guy who rear ends someone is at fault, but since u said middle guy was drunk, u must have police report and they will provide you with the info. Of that person. Call your insurance company, as they will try to help you and then call other guys insurance.
I went through similar situation not too long ago, but in my case it was not drunk driver, just stupid old lady pulled up in the highway of the sudden. Luckily i rear ended her on the side, if it was on the back, it would have been my fault, thats what my agent told me.