The blog module in grid mode seems to be adding padding between rows of posts. To reduce this padding, 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;
}