Xiaomi Mi3 is going to get Android 5.0 Lollipop. @Ivan秋叶随风 (who is a developer working in Xiaomi) is currently working on the Android 5.0 ROM and has posted a picture on Weibo showing MI3 running stock Android 5.0 lollipop. He is the same guy who made the Android 5.0 ROM for the Xiaomi Mi2 and Mi2s.
The ROM is for the WCDMA variant of the Mi3 which is sold out side of China in countries like India etc. There is no word about the public release of the ROM yet, but we can expect to come out in a week or two. Xiaomi is currently work on the MIUI V6 stable version global release. They have already released the MIUI V6 stable version for the Chinese market and will release it for the other markets soon. It's great to see that non nexus devices are also getting the latest Android version this fast.