I know sometimes you want 3 columns of posts even with a sidebar. If that is the case, find this section in the CSS.css file:

/***Two columns for posts***/


/*Set the post width and margin for two columns and add the border*/

.archive .et_pb_post,
.search .et_pb_post {
    width: 47.5%;
    float: left;
    margin-right: 5%;
    border: 5px solid #f5f5f5;
}


/*Remove margin from every second post*/

.archive .et_pb_post:nth-child(2n),
.search .et_pb_post:nth-child(2n) {
    margin-right: 0;
}


/*Set to a single column on smaller screens*/

@media all and (max-width: 767px) {
    .archive .et_pb_post,
    .search .et_pb_post {
        width: 100%;
        margin-right: 0;
    }
}

And replace it with this:

/***Three columns for posts***/


/*Set the post width and margin for three columns and add the border*/

.archive .et_pb_post,
.search .et_pb_post {
    width: 30%;
    float: left;
    margin-right: 5%;
    border: 5px solid #f5f5f5;
}


/*Remove margin from every third post*/

@media all and (min-width: 1280px) {
    .archive .et_pb_post:nth-child(3n),
    .search .et_pb_post:nth-child(3n) {
        margin-right: 0;
    }
}


/*Set to two columns on tablets & smaller laptops */

@media all and (max-width: 1279px) {
    .archive .et_pb_post,
    .search .et_pb_post {
        width: 47.5%;
        margin-right: 5%;
    }
    /*Remove margin from every second post*/
    .archive .et_pb_post:nth-child(2n),
    .search .et_pb_post:nth-child(2n) {
        margin-right: 0;
    }
}


/*Set to a single column on smaller screens*/

@media all and (max-width: 767px) {
    .archive .et_pb_post,
    .search .et_pb_post {
        width: 100%;
        margin-right: 0;
    }
}