#grid-container {
  display: grid;
  grid-template-areas:
    'box-1 box-1 box-1'
    'box-2 box-2 box-3'
    'box-2 box-2 box-4';
  gap: 5px;
}
#grid-container div {
  padding: 20px;
  background: #CCCCCC;
}
#box-1 {
  grid-area: box-1;
}
#box-2 {
  grid-area: box-2;
}
#box-3 {
  grid-area: box-3;
}
#box-4 {
  grid-area: box-4;
}