Unlock -> install TWRP for Android 9 -> Install Android 10 -> Without rebooting into the system install TWRP for Android 10 (find version TWRP for 9T with encryption support in Android 10, it is important! I'm not sure about it, maybe you need to wait for it to be developed for your phone. Сheck...
New TWRP-3.3.1-1015 for MIX3 Android 10 from wzsx150.
Decryption works!
https://dropmefiles.com/hw81c
If you use android 9 with old TWRP, install 9.10.10/11 Rom first, then, without rebooting into Android, install TWRP-3.3.1-1015.
Mi Mix 3
1. Front camera doesn't work.
2. I have friezes (black screen lock, device does not respond, phantom overlays). To fix it - Disable HW overlays in Developer options.