The Divi blog module in grid mode may sometimes add padding between rows of posts. No matter what you do (adjust padding/margins/gutter width, etc on each the section, row, and module) it can be a challenge to get rid of it.

The solution is to add the following CSS code to your custom CSS (Divi > Theme Options > General tab > Custom CSS box): 

.et_pb_gutters1.et_pb_row .et_pb_column .et_pb_blog_grid .column.size-1of3 .et_pb_post:last-child {
margin-bottom: 0 !important;