divs side by side

HTML

<div id="outer">
  <div id = "inner">
    <div class="cont">1</div>
    <div class="cont">2</div>
    <div class="cont">3</div>
  </div>
</div>

CSS

#outer {
    width: 90%;
    height: 90%;
    margin: 5%;
    overflow: auto;
    background-color: red;
}
 #inner {
  margin-right: -20px;
} 
.cont {
    float: left;
    width: 150px;
    height: 150px;
    margin-right: 20px;
    background-color: blue;
}