Well to simply put it, the D3 Cookware products consists of three bonded layer which is made up of stainless steel for its base, an aluminum core for its center material and 18/10 grading stainless steel as the cooling surface.
The D5 Cookware products, on the other hand, is made with 5 alternating layers made up of both aluminum and steel. The base is made up of a magnetic material which is superior enough for induction cooking.
Apart from the materials used to build the two cookware, the main differences lie in the weight, rims, bonded layers, and handles. The All-Clad D5 Cookware Products are weightier than the D3 Cookware Products, but at the same time, the handles feel sturdier than that of the D5.