how can i sign in my iCloud in appletv3?

The legacy Apple TV (3rd generation) [A1427 or A1469] from 2012 unfortunately doesn’t support iCloud services anymore. iCloud has changed almost a year ago, and the legacy system software hasn’t changed with it.

Also see: thread 255664587.

Apple Music (the subscription service) is not supported on that legacy model, and never was — except through AirPlay from a different device (Mac or iPhone or iPad or iPod touch).

Apple Music is supported on Apple TV (4th generation) or later (4K), directly through an on-device app.