Boxing

HTML

<div id="container">
  <div class="box">
    box 1
  </div>
  <div class="box">
    box 2
  </div>
  <div class="box">
    box 3
  </div>
</div>

CSS

#container {
  background-color: #DDD;
  width: 100%;
  height: 60px;
}

.box {
  width: calc(33% - 13px);
  margin: 10px 10px 0 0;
  padding: 10px;
  float: left;
  box-sizing: border-box;
  background-color: #FFF;
}

.box:first-of-type {
  margin-left: 10px;
}