* {
    border:0; margin:0; padding:0; box-sizing: border-box;
}

body {
    background-color: darkgrey;
}

header, main {
    display: flex;
    flex-flow: row wrap;
}

.inner {
    min-height: 33.33vh;
    border: 5px solid black;
    flex-grow: 1; flex-shrink: 0;
}

header .inner {
    flex-basis: 50%;
}

main .inner {
    flex-basis: 33.33%;
}

footer {
    height: 33.33vh;
    background-color: darkkhaki;
    border: 5px solid black;
}

@media screen and (max-width:1100px) {
    main .inner {
        flex-basis: 100%;
    }
}

.inner:nth-child(1){
    background-color: blueviolet;
}

.inner:nth-child(2){
    background-color: brown;
}
 main .inner:nth-child(1){
     background-color: darkgreen;
 }

 main .inner:nth-child(2){
    background-color: chocolate;
}

main .inner:nth-child(3){
    background-color: rosybrown;
}