fill left space

HTML

<div class="wrap">
  <div class="top"></div>

  <div class="mid"></div>

  <div class="bottom"></div>
</div>

CSS

body {
  background-color: red;
  height: 100%;
}

.wrap {
  height: 100vh;
  width: 100%;
  padding: 20px;
  background-color: yellow;
}

.top {
  width: 100%;
  height: 50px;
  background-color: blue;
}

.mid {
  width: 100%;
  height: 50px;
  background-color: green;
}

.bottom {
  width: 100%;
  height: 50px;
  background-color: blue;
}