Mine is 4 1/2 years old and I don't have rust (knock on the wood). I had some superficial rust inside the wheel wells, but the dealer cleaned it up under 5 year corrosion warranty. The body shop put some tape on the lip around the rear wheels.
I'd buy another Mazda3 without hesitation. It's just that I'm not warming up to the exterior design. The interior on the other hand is great (design, quality, features, etc.)
So if I understand correctly, your Mazda3 doesn't have rust after the dealer removed some rust and the body shop protected all the vulnerable spots with tape? Wow... that's insightful!
Maybe Mazda should put clear tape on all their cars! There, rust problem solved! 
No, you didn't quite understand.
I was saying that on mine, I only had
superficial rust, but not on the painted surface and the dealer (Mazda) fixed it under warranty. This was in response to another poster who stated that in his friend's case, the dealer refused to fix it under warranty. I had this done last year and the rust did not show up yet in the same place or elsewhere on the car.
Also, the tape I was talking about is a special kind of tape used in such applications in body shops; it's not the 3M clear film type and it's spray painted to the car colour. I had that added around the rear wheel lip, although no rust was visible.
If you look at the most new cars, they have some sort of "bump" running around the wheel opening, on the inside lip, which I'm sure it has exactly the same role - to finish the lip and protect it from rusting. The new Mazda3 has exactly the same treatment, plus if I don't mistaken, the inside of the rear wheel well is fully painted versus bare metal on the previous generation. This makes me think that the new Mazda3 will not have the same rust issues, at least in this specific area.
If Wing can take a picture of the rear wheel well and the rear wheel opening lip, you could see what I'm talking about and perhaps get over the preset idea that if it's a Mazda it will inheritable rust prematurely.
