There are a guide for Downgrade from android 9 to 8 Downgrade from android 9 to 8 in https://forum.xda-developers.com/t/mi-box-3-mdz-16-ab-downgrade-android-9-pie-to-android-8-oreo.4194075/ .Howerver the method that used in the guide may not work on some mi box 3 . If that method did not work on your Mi box 3 , then you can try the another method that made by exkc
0. Found a usb stick , plug it into a pc and Format it as fat32
1. Dowload this zip https://mega.nz/file/gh0xCCrR#ctdPivezVFNAuMLq19eOepO1LHJCfVNhowmZz7Fx87s
2. Extact update.zip and xiaomi_update from the zip file and put it as the root of usb stick
3. Extact recovery.img from update.zip and put it as the root of usb stick
4. Unplug the usb stick from PC and DO NOT PLUG IT IN TO MIX BOX 3
5. Power on the mi box and boot it into reovery mode
7. Wait util you saw the reovery mode boot up and plug into usb stick
6. Select Apply update fro EXT in the reovery mode
7. Select from udisk
8. Select update.zip
9. Wait util it has flash the zip
10. After reboot you should saw the android 8 uboot has been flashed to the mi box but the android 8 didnt flashed into the mi box and android 9 is not bootable for the uboot so the android 8 uboot will boot from the reovery.img at the root of the usb stick.
11. The recovery will started to load the file xiaomi_update when it boot. After the it has loaded file , it will started install the android 8 from the update.zip file and it will reboot few time during process(It will show you a progress bar . a text that said upgrading system and a logo via the display ) .
12. After step 11 The android 8 will started boot for it fisrt time and it will failed to boot due to the android 9 userdata isnt compatible with android 8.
13 . After failed to boot. The android 8 will reboot into a recovey menu that have text that said "Cannot load Android system. Your data may be corrupt. If you continue to get this message, you may need to perform a factory data reset and erase all user data stored on this device.".
14. Select Factory data reset from the menu.
15. Select yes from the menu.
16. Done
0. Found a usb stick , plug it into a pc and Format it as fat32
1. Dowload this zip https://mega.nz/file/gh0xCCrR#ctdPivezVFNAuMLq19eOepO1LHJCfVNhowmZz7Fx87s
2. Extact update.zip and xiaomi_update from the zip file and put it as the root of usb stick
3. Extact recovery.img from update.zip and put it as the root of usb stick
4. Unplug the usb stick from PC and DO NOT PLUG IT IN TO MIX BOX 3
5. Power on the mi box and boot it into reovery mode
7. Wait util you saw the reovery mode boot up and plug into usb stick
6. Select Apply update fro EXT in the reovery mode
7. Select from udisk
8. Select update.zip
9. Wait util it has flash the zip
10. After reboot you should saw the android 8 uboot has been flashed to the mi box but the android 8 didnt flashed into the mi box and android 9 is not bootable for the uboot so the android 8 uboot will boot from the reovery.img at the root of the usb stick.
11. The recovery will started to load the file xiaomi_update when it boot. After the it has loaded file , it will started install the android 8 from the update.zip file and it will reboot few time during process(It will show you a progress bar . a text that said upgrading system and a logo via the display ) .
12. After step 11 The android 8 will started boot for it fisrt time and it will failed to boot due to the android 9 userdata isnt compatible with android 8.
13 . After failed to boot. The android 8 will reboot into a recovey menu that have text that said "Cannot load Android system. Your data may be corrupt. If you continue to get this message, you may need to perform a factory data reset and erase all user data stored on this device.".
14. Select Factory data reset from the menu.
15. Select yes from the menu.
16. Done
