CSS - Page divided into columns

.section {
  clear: both;
  padding: 0;
  margin: 0;
}

.group:before,
.group:after {
 content:"";
 display:table;
}

.col {
  display: block;
  float: left;
}

.span_3_of_3 { width: 100%; }
.span_2_of_3 { width: 66.6%; }
.span_1_of_3 { width: 33.3%; }
/* GO FULL WIDTH AT LESS THAN 480 PIXELS */
@media only screen and (max-width: 480px) {
 .span_1_of_3, .span_2_of_3, .span_3_of_3 { width: 100%; }
}

<div class="section group">
  <div class="col span_1_of_3">This is column 1</div>
 <div class="col span_1_of_3">This is column 2</div>
 <div class="col span_1_of_3">This is column 3</div>
</div>