The Mi Mix 3 5G has been mostly ignored by Xiaomi so far, as it remains on Android Pie and MIUI 10, almost eighteen months after it was launched. A new tip indicates the phone will receive the MIUI 12 update later in the year, but Android 10 remains an improbability.
https://www.notebookcheck.net/Xiaomi-confirms-October-MIUI-12-rollout-for-the-Mi-Mix-3-5G-sans-Android-10.469336.0.html
when coming miui 12 update coming Redmi Note 7?
I asked the support too, they said the same to me.