If you're signed in but having trouble connecting, check your system's status page for connectivity alerts, maintenances, or other factors indicating that servers are not fully operational. If there are issues with your system's servers, you might experience connectivity issues until they return to full operation.
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.