.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>