GTK (and I think QT also) has a fairly intuitive system for binding keys to commands: Control 3 is just
"<Ctrl>3". However when you want to bind a key with Shift, things get tricky. I had tried
"<Ctrl><Shift>#", all without success.
To make this particular keybinding work, you have to use the special X11 keysym name for that key. I found permanent url that describes these keys here, and a better list with printable characters here.
So for my example, the proper binding would be
However keybindings like this are very tricky and shouldn’t be used in production applications (this example is a personal plugin for GEdit). This is because keyboards in other countries have different symbols over different numbers. So it’s not a given that Shift-3 is the same as numbersign in every country.