Enabling a developer console in Modern Warfare 3 (MW3) depends entirely on which version of the game you are playing, as the modern reboot (2023) and the original (2011) have very different systems. Modern Warfare 3 (2023 Reboot) 2023 version of MW3 does not have a developer console
/ (Forward Slash): Opens the console with a command prompt started. 🛠️ Important Note for Modern MW3
Standard Key: In titles where the console is enabled, the Tilde (~) key (located below ESC) is the universal shortcut.
Start Modern Warfare 3 campaign. Press the tilde (~) key to open the console. It should appear at the top of the screen.