<div class="box">
  <div class="red"></div>
  <div class="blue"></div>
  <div class="green"></div>
</div>

.box {
  height: 400px;
  width: 400px;
  background: black;
  display: flex;
  flex-direction: column;
  overflow: auto;
}

.red {
  height: 50px;
  background: red;
}

.blue {
  background: blue;
  height: 50px;
  width: 600px;
}

.green {
  height: 50px;
  background: green;
}