One piece cranks are typically a chunk of normal Hi Tensile steel some are Cro-Mo but not many. They are very thin and are prone to bending, and breaking rather easily if you jump and ride alot of street. Two pice incorporates the bottom bracket spindle with the drive side crank arm, and the other arm bolts on. Three piece is a seperate bottom bracket spindle and crank arms. This is typically stronger for most manufacturers use tubular or forged cro-mo for the arm material and the same goes for the spindles. It also gives you a wide range in some cases for b.b bearings and spindles.