How to power ADXL362 with consumption as low as possible
A main MCU with internal LDO and DCDC, which could work between 1.8v ~
  3.6v, but is recommended to work above 3.0v due to current leakage in lower voltage


The data sheet for the ADXL362 shows its performance at a supply of 2 volts but it will happily run at 3 volts. Current consumption is 1.8 uA at 2V and 2.7 uA at 3.3 volts. So, I'd run this device on the same regulator as the MCU.

I'm sure you can find a switcher that has lower quiescent current.