<div class="container">
  <div>Div 1</div>
  <div>Div 2</div>
  <div>Div 3</div>
</div>
html,
body {
  height: 100%;
  margin: 0;
}
.container {
  height: 100%;
  display: flex;
  flex-direction: column;
}
.container > div:last-child {
  background-color: tomato;
  flex: 1;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.