[Xiaomi Mi Max 3 Themes, Apps, and Mods] d4ndr0id:
Alright I figured it out. For those who are new to the Mi Max 3 and are wanting Gcam, this is what you need to do:

Make sure your bootloader is unlocked. Mine was shipped unlocked. I have the 6gb/128gb global version.

https://dl.twrp.me/nitrogen/

Download the latest TWRP (nitrogen is the codename for the mi max 3)

Get ADB 1.4.3 HERE

Get Magisk v19.3 HERE

Extract your ADB folder to an easy location (I used c:\adb)

Enable developer options on phone (settings, about phone, tap on MIUI version 5 times)

Enable USB debugging on phone (settings, additional settings, developer options)

Rename your twrp image file to twrp.img (easier for flashing) and place it in the same directory as your ADB

Also put your magisk-v19.3.zip in the same directory as your ADB

Plug your phone in via USB to your computer

open ADB directory via command prompt and run these commands:

adb reboot bootloader

fastboot flash recovery twrp.img

fastboot reboot

Now here's a tricky part: right when you press enter on "fastboot reboot" get ready to hold down power button and volume up once the phone starts to reboot so it enters the new twrp recovery. If you don't do this and the phone boots back into the rom, it'll over-write the recovery and you'll have to reflash the twrp.img via adb all over again. So make sure you nail getting into the new twrp recovery right after "fastboot reboot".

Once in TWRP, go to advanced, tap on ADB Sideload

Go back to your command prompt that has your ADB directory entered. If you closed it, re-open command prompt and enter your ADB directory

command:

adb sideload c:\adb\Magisk-v19.3.zip

Let it do its thing, you might get a few errors, it's fine. reboot.

Download buildprop editor via google play

Open buildprop, make sure it asks for root access, if it doesn't, you didn't sideload magisk properly

Scroll down to where it says "# ADDITIONAL_BUILD_PROPERTIES" for me it was around line 400

Under "# ADDITIONAL_BUILD_PROPERTIES" paste these lines from page 18 of this thread:

persist.camera.HAL3.enabled=1

persist.camera.eis.enable=1

persist.camera.is_mode=4

persist.camera.stats.test=5

persist.vendor.camera.HAL3.enabled=1

persist.vendor.camera.eis.enable=1

persist.vendor.camera.is_mode=4

persist.vendor.camera.stats.test=5

Tap on save. then tap on the three dots at the top right and go to properties

Then go to permissions

Set permissions to 644 by checkmarking "R" for "Group" and "Others" you'll know you did it right because the number below will change from 600 to 644

Tap on apply, then go back, tap on the disk and go to "save and exit"

Reboot your phone

Before installing Gcam, download the app "Manual Camera Compatibility Test"

Start the test. if all goes green, Gcam will work. If it goes red, something wasn't done right in the above steps.

If all goes green, choose your version of Gcam that you like and enjoy :) There is no option for 1080p 60fps video recording and 4k video recording does not work.

If you've softbricked your phone (black loading screen with the three dots but OS never loads), make sure to sideload magisk. if you try to install the zip the normal way through TWRP, it won't bypass the dm-verity and it'll cause softbrick.

If you did softbrick, here's what I did to my Mi Max 3 back to bone-stock:

Get MiFlash HERE I found version 2017.4.25.0 worked best.

Open MiFlash and select "Driver" at the top and make sure all the drivers are installed

Put your Mi Max 3 in fastboot mode (hold power button and volume down)

Go to THIS THREAD and select the fastboot stock rom which suits your model. I'm using MIUI 10 V10.3.3.0.PEDMIXM because I have the global 6gb/128gb version

Download and extract the tgz file to an easy location

In MiFlash, select the folder of the extracted tgz file and hit "refresh" so it finds your phone while its plugged in via USB

At the bottom right, make sure "clean all" is selected

Click on "flash" and let it do its thing. It'll take a few minutes...and blamo, you're back to stock.