Edit in JSFiddle

<div class="flex-container">
  <div class="flex-item">flex item 1</div>
  <div class="flex-item item2">
    <div class="flex-item2"></div>
    <div class="flex-item2"></div>
    <div class="flex-item2"></div>
  </div>
  <div class="flex-item">flex item 3</div>
</div>

.flex-container {
  display: flex;
  width: 600px;
  height: 600px;
  flex-direction: column;
  background-color: Silver;
}

.flex-item {
  background-color: DeepSkyBlue;
  height: 100px;
  margin: 5px;
}

.flex-item2 {
  background-color: yellow;
  width: 200px;
  flex: 1;
  margin: 5px;
}

.item2 {
  flex: 1;
  display: flex;
}