<div id="block">
  <div class="block"></div>
  <div class="block block--greedy"></div>
  <div class="block"></div>
</div>

// autoprefixer: true

html,
body {
  margin: 0;
  padding: 0;
  font-family: sans-serif;
  font-size: 16px;
  line-height: 1.4;
  color: #555;
  background: #fff;
}

* {
  box-sizing: border-box;
}

#block {
  position: fixed;
  top: 0;
  right: 0;
  bottom: 0;
  width: 200px;
  background: #eee;
  padding: 10px;
  display: flex;
  flex-direction: column;
}

.block {
  height: 100px;
  background: #777;
  margin-bottom: 10px;
  
  &:last-child {
    margin-bottom: 0;
  }
  
  &--greedy {
    height: auto;
    flex: 1 1 auto;
  }
}