If you look under header, you see the header-inside container, then the header-widget-area container, and in there arge the three widgets with an aside tag
#custom_html-2
#widget_sp_image-2
#widget_sp_image-3
So if you want to add these margin to the first and last, the rule would look like
#custom_html-2, #widget_sp_image-3 {margin: 50px 4px 8px 4px !important;}
BUT that would, screw up the layout, as the margin are not part of the width, so the widget would drop on two lines
So you should use padding instead and you should not need important
#custom_html-2, #widget_sp_image-3 {padding: 50px 4px 8px 4px;}