found at DigiDirect
The compact Canon Remote Switch RS-80N3 has a 79cm cable and is a remote shutter release designed to prevent camera shake for super-telephoto shots, macro photography, and bulb exposures. Handy for taking pictures of subjects that are difficult to approach, or to minimize vibration for close-ups and time exposure.
The Remote Switch works just like a shutter button, enabling halfway or complete pressing. It also has a shutter-release lock. With the EOS-3's custom function no. 12 (mirror lockup), camera shake can be further reduced.
Note: Only compatible in still mode. Will not trigger the camera in video mode.