Okay, my Mix 3 now has eu-rom and works great!
This is what I have done, based on the thread on xda (
https://forum.xda-developers.com/mi-mix-3/development/xiaomi-eumultimimix38-11-30v10-9-t3877261)
- I was on China stable ROM (latest). USB debugging enabled/unlocked bootloader
- Flashed TWRP (fastboot)
- Reboot to TWRP by pressing Vol up + power until it boots (first it reboots, but TWRP is loading)
- Factory reset
- Boot into fastboot
- Installed china dev rom with miflash tool (clean all)
- Boot into china dev rom, make sure it works
- Boot into TWRP
- Cleared data/cache/dalvik
- Copied EU rom from PC
- Installed EU rom
- Factory reset from TWRP
- Reboot into system
Mix 3 boots in EU rom just fine. But after setup I rebooted to recovery and cleared cache and dalvik once more.
So now I can finally use my phone (china stable was useless for me)