I have figured out a way to control your Android TV using your Mi Band 3. To do this you need 3 apps
2. Mi Band 2 Func Button
Start by setting up a WiFi remote in Anymote app for your Android / Smart TV.
After setting up, go to tasker, create new Profile → System → Intent Received → Type "mb2_tap1" (not the quotation mark) → Go back → Set Task → Plugin → Anymote → Select the remote and Button. I chose Power, to Turn On/Off my TV.
Go to Mi Band 2 Func Button app, click on single tap, and select "Send Command To Tasker". Choose Taker App in the App option.
Now its done. Single Tap your Mi Band 3 button (When Screen is Off), and the function will be exexuted like TV will turn On or Off.
Tutorial For Same on My YouTube channel.