The ThreeFingerSaluteCtrlAltDelete is a KeyboardClaw. Since the 1980's this claw causes the majority of personal computers to reboot.
In the late 1990's, Windows software could give one pause at this point, but pressing the same claw again would kill things.
WindowsNT asks for this claw just to log in. And for a good reason, see CtrlAltDeleteOnNt
Even Linux supports the claw. The default is to reboot the machine without a prompt, which has surprised those used to CtrlAltDeleteOnNt's behavior. You can change its behaviour by altering inittab. I've heard of it being altered to play sound files (eg of HAL saying "I know you and Frank were planning to disconnect me...and I'm afraid that's something I cannot allow to happen.") in case a windows user accidentally tries to shutdown a linux server.
Anybody observe that the original designer of Control-alt-del is retiring from active coding? Hats off to this guy, chaps... DavidBradley? who at IBM around 1980 designed this key combination.