Mi Mix 3 will be announced on 25th October 2018

Expect ROMs after general release, the phone is also said to support the new 5G network technology, has manual front slider housing sensors and selfie camera and such, 6.4 inch AMOLED screen, near 99.9% Screen to Body ratio, 3D face unlock, dual rear 20mp cameras, 10GB RAM (possible) and likely Snapdragon 845 SoC.