There has been a bit of confusion over the MCU in the past few days in the few AU threads for the Model 3. To be clear there are two seperate systems in Teslas. One to control the on screen system (MCUx or Multimedia Control Unit) and a seperate system to control the Autopilot / FSD (HWx or HardWare for Autopilot).
All Model 3s in Australia have the latest HW3 for Autopilot. This has the chip internally developed by Tesla.
The MCU is seperate. The current MCUs are Intel based. All Model 3s in AU essentially have a repackaged version of the MCU2 found in the latest S/X models. It get technical, but the architecture of the Model 3 is different from the S/X so the MCU units are not the same across S/X and 3 due to handling of multiple screens on the S/X, etc. But from a performance standpoint, they are identical to the MCU2 on all current Teslas.
Here is a good link detailing...
Autopilot, Processors and Hardware – MCU & HW Demystified | TeslaTap
As to future upgradability, Musk has said that the HWx hardware for autopilot will get upgrades if you purchased the FSD (HW4 is currently in development). From the best we can all tell, MCUx will not get upgraded for "free" and it is unknown if the Model 3 will be able to upgrade for a fee in the future if they move to a "MCU3".