Customize text widget's margins individually

Atahualpa rocks!!! One question: I want to change the margins but only on certain text widgets in my sidebar. I know there is an option in:

ΑΤΟ > Sidebars & Widgets > Style widgets > Widget Container but that does the job for all of them.

I want to experiment only with three of my text widget's margins... How do I do that?

Thanks in advance