Castrol has three multivehicle ATFs. The older and identical Import-ATF are synthetic blends and would work great in your Mazda 3.
The Domestic-Multivehicle is simply the normal merc/dexiii equivalent and will also work. But, its not an upgrade.
Also, another option is the Castrol Mercon-V which would also be an upgrade to the domestic-merc/dexiii fluid.
DexronVI, even though a little thinner, should have no problem in PSF systems that require any Dexron or Mercon. But, there might be a change in 'feel' when playing with PS fluid weights. In place of DexVI, any of the full synthetic versions of DexronIII/Mercon should also be considered.