The initial MIUI 11 beta update for the Xiaomi Mi Mix 3 was based on Android Pie, but the phone eventually received a taste of Android 10 via the closed beta channel back in October of last year. The stable version of MIUI 11 for the Mi Mix 3 landed in the beginning of November, but the underlying Android version was still based on Android 9 Pie. Xiaomi dropped a hint about the upcoming OS upgrade when they published the kernel source code of the Android 10 update for the Mi Mix 3, alongside the POCO F1, Mi Mix 2s, and the Mi 8 series in December. The wait is finally over, as Xiaomi has now started rolling out Android 10 for the Mi Mix 3 in the form of MIUI 11.0.3.0 via the stable channel.
The version numbering scheme coined by Xiaomi is enough to confuse most regular users. The last Android Pie-based build for this phone was tagged as 11.0.4.0, thus the newest update apparently looks like a downgrade. The Android Security Patch Level (SPL) is bumped from October 2019 to February 2020 though, which is a significant improvement in terms of security. Apart from the multitude of Android 10 software enhancements, the update also contains fixes for various glitches related to the notification shade as well as overlap issues in scrolling screenshots.
Download MIUI V11.0.3.0.QEEMIXM for the Mi Mix 3 (codename: perseus)
The update is no longer considered a "stable beta". As a result, you don't explicitly need an "authorized Mi Account" to sideload this build. People with an unlocked bootloader can also flash the Recovery ROM via TWRP or opt for the Fastboot package to perform a clean install.
It is worth mentioning that the Mi Mix 3 5G (codename: andromeda) is still running MIUI 10 on top of Android Pie. The aforementioned firmware is not at all compatible with the 5G variant, so do not try to force-flash this build unless you want a costly paperweight for your desk.
Source: Mi Community
Thanks to XDA member mattstifler for the screenshot!