Ok now I understand what you’re saying. You just want to add a background image to the home feature 3 only.
The reason the picture of the sky is showing up on all of your pages is because you added it to the main body/background. I would remove that image, and directly add it to the .hf3 code that Wes mentioned earlier. By doing that, it will only show up on the home-feature-3 widget area.