If you still have the Redmi 3S or the Redmi 3S Prime handset check for update notifications, there probably might be a new update, the MIUI 126.96.36.199. If you haven’t got the update wait for few more days and then check again.
After announcing MIUI 9 Xiaomi had given the list of Smartphones that will get the MIUI 9 update and as promised the updates have already started rolling out. This MIUI 9 will make the overall experience even smoother.
Now for older models the MIUI 9 like in the case of Redmi 3S and Redmi 3S Prime, its MIUI 188.8.131.52 based on Marshmallow. This is nightly build which means only some users will get it and the update is rolled out using random method.
Once the company feels the update is good enough then the same will be rolled out to other users. MIUI 9.X will bring new layout, MIUI 9 features such as dual apps, Mi Drop etc., the new MIUI 9 is much faster than the MIUI 8 and hence after installing the MIUI 184.108.40.206.MALMIEK on the Redmi 3S or the Redmi 3S Prime, it should improve performance and also have some impact on the battery life.