Consumer Reports said the Mazda3 had excellent fuel economy, so I would take the claims otherwise with a grain of salt. Brakes do appear to have more problems in the first year than the average car, but is offset by much lower repair rates in other areas.
I hated how the Grand Prix drove. It's a shame when the older design is a better car (handling, fit and finish, room, comfort, and style).
I drove the Scion Tc and like you said, found the handling to be subpar. It drives like a much larger more conservative car. Headroom is also tight.
I liked the 3... I don't know what you mean about the 6 being too "thin", but the 3 has a lot going for it. Probably the nicest car for the money.
