SOURCE

  <section>
    <div class="red"></div>
    <div class="yellow"></div>
    <div class="blue"></div>
  </section>

html {
  height: 100%;
}
body {
  height: 100%;
}
section {
  display: flex;
  flex-direction: column;
  align-items: stretch;
  height: 100%;
}
.red {
  background-color: red;
  height: 50px;
}
.yellow {
  background-color: yellow;
  flex-grow: 1;
}
.blue {
  background-color: blue;
  height: 50px;
}