[Xiaomi Mi Mix 3 Guides, News, & Discussion] reasonablebasis:
Disclaimer: I am not a developer (I don't code), but I have been flashing ROMs for many devices over the years. This is more of a practical guide from an end-user perspective in pursuit of finding the best ROM for this magnificent hardware. This is more of a compilation of all the relevant information I found. I'm very open to suggestions/corrections that could improve this.

Disclaimer: Only tested with RR, PixelExperience, and ArrowOS GSI.

I haven't seen a proper guide to flash GSI ROMs for Mi Mix 3, thus I decided to share what I know that works. I put a brief explanation for each step below.

1. Unlock Bootloader - If you haven't done so yet, I recommend following the instructions in https://www.reddit.com/r/Xiaomi/wiki/bootloader

2. Install TWRP

a) I recommend to use the official TWRP from this link https://twrp.me/xiaomi/xiaomimix3.html or refer to the forum https://xdaforums.com/mi-mix-3/development/recovery-unofficial-twrp-recovery-t3901261.

b) To install TWRP, I recommend to follow the instructions from Xiaomi.eu. For those not familiar, the "twrp.img" in the code should be the full path of the file. Something like "D:\folder\twrp.img"

c) If you have an older version of TWRP, I suggest to update it to the official.

3. Install Xiaomi.eu - Start from this step in case of bootloops or issues (especially caused by Magisk modules by my experience)

a) This is what I do instead of the 'flash vendor.img from Chinese Dev' prerequisite mentioned in other threads. This is an easier step (just one flash in TWRP) and a better fallback (a working ROM) in case things go wrong. I have had no issues (fingerprint, calls, etc..) with this and I've done this thrice.

b) Download the latest MIUIv10.2 Stable for MIMix3 from the link https://xiaomi.eu/community/threads/miui-10-0-10-1-10-2-stable-release.47170/.

c) Wipe > Format Data (prevents problems, and removes encryption if you had it). Also do Factory Reset (idk, just to be sure).

d) Copy xiaomi.eu ROM to internal storage, then Install > Flash. No need to boot/restart, proceed to the next step.

4. Install GSI ROM - wth is GSI? https://github.com/phhusson/treble_experimentations/wiki/Frequently-asked-questions-(FAQ)

a) We can actually install any GSI in https://xdaforums.com/project-treble/trebleenabled-device-development but there is an issue in installing Google Apps (Open GApps) as many people mentioned in https://xdaforums.com/mi-mix-3/development/9-0-resurrection-remix-v7-0-t3891975 and as confirmed by the official TWRP thread https://xdaforums.com/mi-mix-3/deve...p-recovery-t3901261/post79035909#post79035909

b) Given the above, the only feasible options are GSI ROMs with built-in GApps and the only 2 I found are PixelExperience GSI and ArrowOS GSI. As GApps is integrated in the system.img, no need to flash Open GApps thus no issues. Having tried both, I personally recommend ArrowOS due to the performance and additional customization options.

c) Download the arm64 AB version (ArrowOS as an example) https://xdaforums.com/project-treble/trebleenabled-device-development/gsi-arrowos-t3835111

d) Copy the resulting system.img to internal storage (extract if its a .zip), in TWRP click Install > Install Images > select system.img of ArrowOS > select System partition. You can now boot, or follow next step to root with Magisk.

5. Install Magisk

a) The Magisk version that passes SafetyNet is the builds from ianmacd https://github.com/ianmacd/MagiskBuilds look for v18.2 (newer versions might work but I haven't tried them yet). You can still use the official Magisk Manager and the ianmacd version will be recognized. Just change the update channel in Magisk to https://goo.gl/yZpnrf. Credits to this thread https://xdaforums.com/mi-mix-3/how-to/root-magisk-18-2-mix-3-t3905754

b) After flashing Magisk, boot, install Magisk Manager, then download and install the module "SafetyPatch". SafetyNet check will now be success.

c) Even though SafetyNet is passed, some banking apps still are not working. Hopefully someone can share a solution (probably MagiskHide Props Config, or canary builds?) to make banking apps work.

Final Thoughts

ArrowOS GSI seems to be stable enough and I'm using it as my daily driver now. I am so much happier compared to when using MIUI Global ROM. I really like the performance (very fluid), battery life (a lot better than MIUI Global), customizations (like traffic indicator, status bar stuff), unli Google Photos, and everything that makes AOSP/stock android awesome. No more bloatware, annoying non-dismissable notifications, and horrible auto brightness from MIUI.

The only real issue I encountered so far is the bluetooth audio/music issue, but this seems to be a common problem in GSI ROMs. Bluetooth calls work though. There is no app to handle the slider (but I don't care. lol). Lastly, the AI button becomes useless, but see below for the fix.

Optional stuff that makes things better:

1. Remapping AI button - When using GSI ROMs, AI button becomes useless. But to map it back to Google Assistant (or anything actually), follow the instructions from this thread (needs some knowledge about Tasker). https://xdaforums.com/showpost.php?p=78523778&postcount=38 For the Tasker Task, I selected Input > Voice Command. This now assigns the button to open Google Assistant. Does not work well when screen is off though, but its just easy to unlock.

2. Google Camera - The main reason why I bought this phone. Google Camera brings out the best in the awesome camera - Use this version https://xdaforums.com/mi-mix-3/themes/gcam-6-manual-focus-long-exposure-t3885610

3. Stereo Speaker Mod - https://xdaforums.com/mi-mix-3/themes/magisk-stereo-speaker-mod-mix-3-t3900580 Makes the phone earpiece into a speaker, increasing overall volume. I personally use version 3 and I'm very happy.

4. Fluid NG - App in Google Play Store to have the same recents/home/back gestures from MIUI 10.