.contenedor {
    width: 100vw;
    height: 100%;
    display: grid;
    grid-template-areas: 
    "header      header      header"
    "section    section      aside"
    "section    section      aside"
    "footer      footer      footer";
    grid-template-rows: repeat(3, 500px) 80px;
    grid-template-columns: repeat(3, 1fr);
    gap: 10px;
}

.caja:nth-child(1) {
    grid-area: header;
}

.caja:nth-child(2) {
    grid-area: section;
}

.caja:nth-child(3) {
    grid-area: aside;
}

.caja:nth-child(4) {
    grid-area: footer;
}