If you were the 3rd driver in a rear end accident where the car in front of you hit the car in front of it and you hit the 2nd car who is at fault if you didn't see their brake lights come on?

you are At Fault of the vehicle you hit, it is possible you were following too close, the other veh is at fault of the veh they hit and you are at fault of vehicle you hit. Insurance and accident investigators generally use a method referred to as "chain of causation". During the investigation they determine which car was the last car in motion and then work from that point. Using this process in regards to multiple rear end collisions they are usually able to discover which driver made the first violation or error that created the accident. That means that the driver of a vehicle that struck another one from the rear is not necessarily the one at fault.