Edit in JSFiddle

.wrapper {
	width: 960px; /* Some arbitrary bounding. 960px is usually the width of fixed-width layouts. */
}

.three-column {
	width: 33%; /* 100%/3 = ~33% */
	height: 200px; /* Stick in an arbitrary height for display purposes. */
	float: left;
	background: #f00; /* Red, again for display purposes */
    padding: 0 5px;
}

.three-column:first-child {
	background: #0f0; /* Green, to show the column differences */
}

.three-column:last-child {
	background: #00f; /* Blue, to show the column differences */
}