RE: Mazda 3 tire cupping
The Mazda3 does have negative camber in the rear by design (that's partially how you get 0.87g skidpad readings in an economy car), this causes the tires to wear on the inside edge. Tire rotation is the only real solution without affecting your handling.
Typically a camber kit is used where someone has lowered the car and put the camber even more negative. (Mine went from -1 to -2.1 with a minor drop) I installed the Mazdaspeed camber kit and was right back at -1)
With regular rotation I got 42,000 miles out of Toyo Proxes FZ4's before I needed to replace them going into winter
2004 Mazda6 Sportwagon
Any statements made are purely my opinion