index3.css

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

.container {
    height: 100vh;
    display: grid;
    grid-template-areas: 
    'header header header'
    'nav content sidebar'
    'nav footer footer';
    grid-template-columns: 1fr 4fr 1fr;
    grid-template-rows: 80px 1fr 70px;
}

.container__nav, .container__main, .container__header, .container__footer, .container__aside {
    background-color: steelblue;
    border: solid 1px #ffffff;
    padding: 20px;
}

.container__header {
    grid-area: header;
}

.container__footer {
    grid-area: footer;
}

.container__nav {
    grid-area: nav;
}

.container__aside {
    grid-area: sidebar;
}

.container__main {
    grid-area: content;
}