.container_3box {
    display: flex;
    flex-wrap: wrap;
    background-color: #edeffd;
    }
    
.box {
    width: calc(33.3% - 20px);
    margin: 10px;
    padding: 10px;
    background-color: #fff;
    text-align: center;
    }

@media screen and (max-width: 768px) {
    .container {
        flex-direction: column;
    }
    .box {
        width: 100%;
    }
    }