I use ion3 window manager in debian distribution and I was wondering what keys combination I need to use to take a snapshot of my screen to paste it in gimp for instance. Any help will be very appreciated.
There is no such thing, unless ion3 has been extended to do something that isn't the job of a window manager.
You can use an external utility such as import (from imagemagick) to take screenshots and save to a file. You could bind a key in ion3 to run it, of course.
Gimp should also be able to take screenshots for you by itself. Try File/Acquire/Screenshot (or the like, one of the first items).