<div class="container">
  <div class="main"></div>
  <div class="left"></div>
  <div class="right"></div>
</div>
* {
  margin: 0;
  padding: 0;
}
.container {
  display: flex;
}
.main {
  flex-grow: 1;
  height: 300px;
  background: red;
}
.left {
  order: -1;
  flex: 0 1 200px;
  margin-right: 20px;
  height: 300px;
  background: blue;
}
.right {
  flex: 0 1 100px;
  margin-left: 20px;
  height: 300px;
  background: green;
}

External CSS

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External JavaScript

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