Today Xiaomi has started rolling out the update to Android 10, with MIUI 11 on top, for the Mi Mix 3 from 2018. We're talking about a global stable version, no "beta stable" shenanigans here, and no Chinese-only stuff.
The update is currently out for "other global regions", that is, ones that are not India, Indonesia, Russia, or the EU - as these have their own firmware codes. Still, it's a start, and hopefully we'll see the update make it to all "global" regions (in Xiaomi parlance) soon.
If you're lucky enough to have the "other global" ROM on your Mix 3, then expect to see a notification that will ask you to install MIUI 18.104.22.168 (QEEMIXM) on your phone. It's a ~2GB download, and once you have it up and running you're also getting the February 2020 security patch level, as well as a bunch of bug fixes and various performance optimizations.