Three-finger salute is slang used to describe the IBM PC compatible three key sequence Ctrl+Alt+Del that was developed by David Bradley. This term became widely used during a time when PC users were frequently pressing these keys each time their computer froze. It is still often used to restart a computer that gets stuck in the boot process or cannot load the Windows operating system.
Today, when pressing Ctrl+Alt+Delete, an NMI is sent to the CPU and Microsoft Windows opens a full-screen menu. The menu gives access to Lock the computer, Switch users, Sign out, open the Task Manager, and Shut down the computer. Earlier versions of Windows only opened the Task Manager, which allows them to see what's running on the computer and have access to End Task open programs.
Is it written as "Alt+Ctrl+Del" or "Ctrl+Alt+Del"?
Although you'll get the same results when pressing these keys in any sequence, it's more common to use "Ctrl+Alt+Del" or "Ctrl+Alt+Delete" in your writing, over the phone, or in person.