<div id="wrap">
  <div id="first">1</div>
  <div id="second">2</div>
  <div id="third">3</div>
</div>

#wrap {
  display: flex;
  width: 900px;
}  
#first {
  flex: 1 200px;
  background: #f00;
}
#second {
  flex: 2 50px;
  background: #0f0;
}
#third {
  flex: 3 50px;
  background: #00f;
}