The Mi Mix 3 5G was Xiaomi's first 5G phone and was powered by the flagship Snapdragon 855. In spite of that, it's been mostly neglected and remains on Android Pie and MIUI 10. Android 10 is unlikely to arrive for the phone, too, but like with the Mi Mix 2, MIUI 12 may be on the cards.
https://www.notebookcheck.net/Android-10-for-the-Mi-Mix-3-5G-and-Mi-Mix-2-remains-a-pipe-dream-but-MIUI-12-could-be-on-the-cards.466846.0.html