Changing Shadow darkness

I am drawing 3 shadows(Simple shadows) of an object using 3 light sources.The darkness of all shadows is constant( using glColor3f).
I want to change darkness of shadows whenever light sources moved.
Please Help.

When rendering for each light source, stencil out the shadowed area for that light, and blend the lighting in. You can end up with the lighting for 2 lights in a shadowed area, but not having the lighting for the light that is casting that shadow.