Xiaomi is pushing the stable MIUI 10.3.2.0 firmware update to both the Mi Mix 2S and the Mi Note 3 but there are differences. Key among them is the OS version - while the Mi Mix 2S moves up to Android 9.0 Pie the Mi Note 3 is left at Android 8.1 Oreo.
Some of the major improvements of the firmware are the March 2019 security patch and the addition of Face Unlock support (region dependent).
There are a number of bug fixes and patches as well. Here's a reminder of what's new in MIUI 10 and what's coming in MIUI 11.
For firmware links and instructions you can visit the XDA Forums in the Source link below.