The Chinese technology company revealed the prototype in a post to the social media site Weibo.
It promised the "double-folding" device was coming and asked the public to help name it.
Samsung and the start-up Royole have previously unveiled flexible-screened phones of their own.
But they only folded at a single point.
"It is very interesting that Xiaomi has adopted a three-way folding option on its prototype," said Ben Wood, from the consultancy CCS Insight.