p {
    padding: 20px;
}

.row { 
    display: flex; 
    flex-flow: row wrap; 
    justify-content: space-between;
} 
.row:after { 
    content: ''; width: 100%; 
} 
.box { 
    flex: 1; 
} 
.box:nth-of-type(n + 4) { 
    order: 1; 
}

:host {
    margin: auto;
}