Use same input node values for different materials

I have an object with 3 materials and I want to set the RGB color equal, but only using one set of 3 values (R,G,B) defined as input values. I want to do that in order to expose in driver to use when linked .

I have read a post about using a nodegroup (post), I tryed but I'm unable to get work.

Here a test file: http://pasteall.org/blend/index.php?id=47846

What can I do?