Xiaomi may have teased the third generation Mi Band 3 with a new image that was discovered up on the web. The teaser image which was just posted by Xiaomi mentions an April 27th date and shows a person exercising with what looks like a wearable on their wrist. This isn't a confirmation of anything at this point as the image was altered so that you can't actually see the wearable's design, and it's entirely possible that the teaser may not be for the Mi Band 3 at all but a completely different wearable device from the company instead
So far there doesn't seem to be much known about the Mi Band 3 due to a lack of rumors surrounding the device, but it tends to be expected that companies such as Xiaomi would continue to launch new versions of their popular products, which is why they're thought to be moving forward with a third generation Mi Band fitness tracker. Xiaomi's previous generation of the wearable, the Mi Band 2, was officially announced back in June of last year, so if this teaser for April 27th is for the Mi Band 3 then that would make it so Xiaomi is looking at an earlier announcement this time around. Having said that, Xiaomi's CEO showed off the Mi Band 2 in an image online in April of last year, so an official April announcement wouldn't be that surprising.
There is literally no detail about the Mi band 3 at this point and no confirmation that it's been in the works at Xiaomi, but it seems clear that the company does have at least some sort of wearable ready to present to the public in the near future, and while the Mi Band 3 may very well be the most likely scenario it can't be ruled out that they may have something else up their sleeves. Xiaomi also recently held their official event to announce the Mi 6 which is now their most recent flagship smartphone, and this followed an unveiling of the Xiaomi Mi Band 2 Mi 6 Commemorative Edition that was designed in celebration for the Mi 6 launch, which at the time had still yet to happen.