.item {
  background: lightblue;
  margin: 2px;
}

.item2 {

  background: lightblue;
  margin: 2px;
}

.main-container {
  display: flex;
  width: calc(100vw - 16px);
  height: calc(100vh - 16px);
  justify-content: strech;
}

.main-container div.container {
  flex-grow: 4;
  height: 100%;
  display: flex;
  flex-direction: column;
}

.main-container div.first-container {
  flex: 1.5; /* because in the picture you posted the first box was a bit larger */
}

.main-container div.container div {
  flex-grow: 1;
}