#x3se1 {
    padding: 20px 0;
}
#x3d1 {
    display: flex;
    flex-wrap: wrap;
    width: 1000px;
    justify-content: center;
    margin: 0 auto;
}
.x3i1 {
    margin: 20px;
    width: 240px;
}

/* media quaries */
@media screen and (max-width: 1200px) {
    #x3d1 {
        width: 98%;
    }
}
@media screen and (max-width: 900px) {
    .x3i1 {
        width: 35%;
    }
}

@media screen and (max-width: 600px) {
    .x3i1 {
        width: 55%;
    }
}

@media screen and (max-width: 450px) {
    .x3i1 {
        width: 65%;
    }
}