Brake issue

I am facing problem in braking and for the braking issue I visited service center. They firstly replaced both the discs and brake pads of front wheel as the problem was left side disc is totally heated. But after replacing both disc and brake pads after sometime left side brake pad totally wear out as compare to right one. Then they replaced caliper of left side with new set of front brake pads. But after replacing this I noticed in a long drive that brake pedal is going down and car is running heavy after coming. I noticed that right side of brake pad and disc totally wear out as compare to left. I would like to know the actual cause.