Because Google has released the Android 6.0 code to AOSP, developers around the world are free to download the code, and customize it for a given device and come with a custom ROM. Something similar is going to happen soon with Xiaomi Mi3, whose Marshmallow update is already under the works.
You can see the screenshot at the top and bottom of this article as a proof.
It’s not unfair to say that other top Xiaomi devices could follow the lead, as development is pretty similar for the company’s devices.
We could be seeing Android 6.0 based custom ROMs appearing for more popular sets like Xiaomi Mi4, Mi5, Mi4i, Redmi Note 4G, Note Pro, etc.
It’s clear that development builds are already out for the Mi3, and here’s a hoping that final builds would be good daily driver, something you can easily live with without compromising on necessary features.
Anyway, keep your Mi3 ready with TWRP recovery installed, a Marshmallow build could drop anytime now. Keep watching this space for that.