You can add CSS to the install to change the default settings from 4 columns to 3 columns in the Divi Portfolio module.
There is a great tutorial on it here
Add the following class to the module advanced CSS settings… three-column-grid
Then add the following code to the CSS area of Divi.
/* THREE COLUMN PORTFOLIO GRID LAYOUT */
@media only screen and ( min-width: 768px ) {
.three-column-grid .et_pb_grid_item {
width: 28.333% !important;
margin: 0 7.5% 7.5% 0 !important;
}
.three-column-grid .et_pb_grid_item:nth-child(3n) {
margin-right: 0 !important;
}
.three-column-grid .et_pb_grid_item:nth-child(3n+1) {
clear: left;
}
.three-column-grid .et_pb_grid_item:nth-child(4n+1) {
clear: unset !important;
}
}