MIUI 6 update is now available for your Xiaomi MI 3 and Xiaomi MI 4 smartphones. For downloading this update for your smartphone, you can simply click the download link provided in the link given below for your smartphones respectively.
The update comes with many new features for both the smartphones like the emojis, wireless printing, full screen album art, security and bug fixes. The size for MIUI 6 update of Xiaomi MI 3 is about 333MB for stable version and for Developer ROM version the size is 334MB. The size for MIUI 6 update of Xiaomi MI 4 is about 424MB for stable version and for Developer ROM version the size is 417MB.
For either of your smartphones the links are given above. You can follow the flashing guide given on the download page to flash the ROM on your Xiaomi phones.