Add the class hws_3col_portfolio to the modules advanced properties

and then add the following css to the Divi theme options.

/* START 3 column portfolio */
@media screen and (min-width: 767px) {
.hws_3col_portfolio .et_pb_portfolio_grid_items {
display: flex;
flex-wrap: wrap;
justify-content: space-between;
}
.hws_3col_portfolio .et_pb_portfolio_grid_items > div {
width: 30% !important;
margin: 0 !important;
}
}
/* END 3 column portfolio */
Code language: CSS (css)