Updating Multiple Ports on 8bit AVR in Single Cycle
No.

The AVR has an 8bit data bus and that means it can only write to one 8bit register at a time. There is no way of programming three bytes at the same time.



If you are accessing that much memory, I would suggest you consider going for a 32bit MCU. It is possible to get 32bit AVR Processors if you want a similar CPU. Alternatively there are many relatively simple/cheap 32bit ARM processors from many manufacturers. Take your pick.