Xiaomi has just released the MIUI global beta build running on Android Marshmallow for the Mi 3, Mi 4 and the Mi Note. The global beta build carries the build number 6.3.17.
There are a lot of improvements under the hood including a permission manager, shortcuts to hidden folders, card layout in explorer and then there's everything including Doze mode for better battery life with Marshmallow innards.
If you are already on the global beta ROM, you can just use the updater app to download the new update. For everyone else you would have to install the update manually and you can check this link for more information.
At present there is no word on a stable ROM release.
