Before adding this widget, you Install and Activate Image Widget plugin (as recommended by my theme notice box).
Go to Appearance > Widgets and find “3 Featured Images header” tab.
Find “Image Widget” widget and drag or put it under “3 Featured Images header“, add only 3 widgets, so it will look like this image bellow :
After you add the widget, click the widget (click the triangle), upload Image (your image size has to be larger than 1024px width and 1024px height).
Fill in the following input boxes :
Repeat for the rest of other widgets.