If when I was going down the interstate and the car in front of me hit the rear-end of the car in front of them causing me to slam on my brakes and I then in return hit them in the rear, who is at fault?
There are two drivers at fault, who can both be liable for the damages from the accident, based on what you write: the car in front of you, who rear ended the car in front of it; and you, who rear ended a car, too. The law presumes that any driver who rear ends another car is at fault, since they are obligated to keep an appropriate following speed and distance, and to pay sufficient attention as to be able to react promptly, so as to not hit a vehicle in front of it if that vehicle stops short any any reason (e.g. debris, accident, pedestian, deer, large pothole, mechanical failure, etc.). Almost always, if you hit the car in front of you because it slowed or stopped suddenly, you were at fault, since you should have maintained a speed and distance so that if they stop, you can stop, too.
In the situation you describe, the only not-at-fault driver is the third one, the one who was hit from behind by the car in front of you.