Change padding of a few selected widgets to 0

Hello,

I'd like to change the padding of 3 widgets on the right sidebar to 0px. I have a total of 6 widgets. The 3 widgets in question are showing ads and so, I'd like to utilize maximum space for them. All three widgets are custom HTML widgets and I tried the following CSS code but didn't work.

#custom-html-3 {
padding: 0;
}

It works for other widgets though but doesn't work for Custom HTML widget with AdSense ads.