Edit in JSFiddle

Result
CSS
HTML

.container {
  width: 200px;
  height: 200px;
}

.container > div {
  width: 20%;
  margin-left: 20%;

  height: 20%;
  margin-bottom: 20%;

  float: left;
  background: red;
}

.container > div:nth-child(3n+1) {
  margin-left: 0;
  clear: both;
}

<!DOCTYPE html>
<html>
<head>
    <title>CSSBug</title>
</head>
<body>
    <div class="container">
        <div></div>
        <div></div>
        <div></div>
        <div></div>
        <div></div>
        <div></div>
        <div></div>
        <div></div>
        <div></div>
    </div>
</body>
</html>