There were reports a while back that the Mi Mix 3 5G wouldn't receive any other update but those may have been a tad misguided, as Xiaomi has now confirmed that the last-gen premium phone is set to get a new update shortly, with MIUI 12 to follow.
https://www.notebookcheck.net/Xiaomi-Mi-Mix-3-5G-finally-primed-for-a-new-update-as-MIUI-12-looms.483902.0.html