The Xiaomi Mi Mix 3 may not receive MIUI 12 until September. The device, which Xiaomi released in October 2018, is on Round 2 of the company's MIUI 12 Release Schedule. However, it is not eligible for its fledging Mi Pilot progam.
https://www.notebookcheck.net/Stable-MIUI-12-update-may-not-arrive-on-the-Xiaomi-Mi-Mix-3-until-September.480308.0.html