[Android Stick & Console AMLogic based Computers] VivaLaVent:
This is a copy of the comment I made in the [Help] topic, figured I would make a separate post since that thread is a mess. Anyway, credit for the method is to krytikul and evelpotatoman. I am not responsible for any damage/unwanted results/house fires. Use at your own risk yada yada.

Without furher ado, here is my tutorial for those who want working SuperSU on their MDZ-16-AB Mi Box 3. Warning: since the bootloader isn't unlockable yet, you will have to connect a laptop to run some commands EVERY TIME YOU BOOT UP THE MI BOX. If you do not connect to run the specified commands YOUR DEVICE WILL BOOTLOOP, it won't even be able to boot in unrooted state. Luckily, this doesn't apply to waking up from standby mode, so as long as you keep your Mi Box plugged in and don't power it off, you'll be fine.

You will need:

A USB male to male cable (I made one myself soldering two cable plugs together)

Mi Box Marshmallow userdebug firmware ROM (this guide will not work if you are already on Nougat test build)

SuperSU installation files

The ability to use adb on your computer

The steps:

Put the downloaded userdebug firmware ROM on a FAT32 formatted drive.

Enable developer options on the Mi Box. This can be done in your settings, find "About" and then click on "build" 7 times to enable the developer options. It will confirm you've done this with an on-screen toast notification.

Connect the Mi Box to your computer and check if a device comes up when typing "adb devices" in command prompt. If not, wait for the Mi Box to display a dialogue confirming you want to allow adb on this computer.

Type "adb reboot recovery" to put the Mi Box in recovery mode. Once in recovery, unhook the cable and plug in your usb, click "Apply update from EXT" then "Update from udisk" and select MiBOX3_USA_once_r454.zip, then just let the update do it's thing.

Reboot the device. Once we're back, see if the box is still under adb devices.

Now run these commands:

adb root
adb remount
adb disable-verity
adb reboot

The device will reboot. Once it's rebooted, run these commands to disable OTA updates (otherwise it will immediately update to an unrootable firmware

adb root
adb remount
adb shell pm disable com.google.android.gms/.update.SystemUpdateService
adb shell pm disable com.google.android.gms/.update.SystemUpdateService$ActiveReceiver
adb shell pm disable com.google.android.gms/.update.SystemUpdateService$Receiver
adb shell pm disable com.google.android.gms/.update.SystemUpdateService$SecretCodeReceiver
adb shell pm disable com.google.android.gsf/.update.SystemUpdateActivity
adb shell pm disable com.google.android.gsf/.update.SystemUpdatePanoActivity
adb shell pm disable com.google.android.gsf/.update.SystemUpdateService
adb shell pm disable com.google.android.gsf/.update.SystemUpdateService$Receiver
adb shell pm disable com.google.android.gsf/.update.SystemUpdateService$SecretCodeReceiver
adb reboot

Wait for the device to reboot. Now we've disabled OTA updates we can install SuperSU. Unzip the SuperSU installer and run the installer BAT script. When the script is done, it will seem as if the device is bootlooping. From now on, this will happen EVERY TIME THE DEVICE BOOTS. If you do not connect to run the specified commands YOUR DEVICE WILL BOOTLOOP, it won't even be able to boot in unrooted state. Below are the steps you should perform to get your device to boot in SuperSU-rooted state.

While the bootanimation is running, keep executing "adb devices" until your device is in the list.

Once the Mi Box is in the adb devices list, type "adb root" and press enter. your device will be disconnected from adb again.

Keep executing "adb devices" again until the Mi Box reappears. When it's there, type "adb shell setenforce 0"

All these steps are performed while the bootanimation is playing. The device will boot shortly after the last command is executed. Be patient as it can take a while for the device to re-enter the adb devices list.