SOURCE

<div class="content">
  <div class="left">left</div>
  <div class="main">content</div>
  <div class="right">right</div>
</div>

* {
  padding: 0;
  margin: 0;
}
.content {
  width: 100%;
  display: flex;
}
.main {
  flex: 1 100px;
  background-color: yellow;
}
.left {
  width: 100px;
  background-color: red;
}
.right {
  width: 100px;
  background-color: orange;
}