Yes, Black Ops 3 is still active on Xbox One. Despite being released in 2024, the game continues to have an active player base on the Xbox One platform.
As of writing this article, Call of Duty BO3 servers remain live across all platforms, which includes PC, PlayStation 4, Xbox 360, Xbox One, Mac, and PlayStation 3. Its multiplayer servers still undergo periodic maintenance and continue to be fully functional in 2024.