<ul>
  <li>1</li>
  <li>2</li>
  <li>3</li>
  <li>4</li>
  <li>5</li>
  <li>6</li>
</ul>

@import url(https://fonts.googleapis.com/css?family=Lato:900);

* {
  box-sizing: border-box;
}

body {
  font-family: 'Lato', sans-serif;
  /* color: white; */
  font-size: 48px;
}

ul {
  width: 50%;
	margin: 20px 0;
	padding: 0;
	list-style: none;
	display: flex;
	flex-flow: row wrap;
}

ul li {
	border: 10px solid #27ae60;
	text-align: center;
	flex: 1 0 calc(33.33% - 20px);
	padding: 20px;
	margin: 0 10px;
}

ul li:nth-last-child(n+4) {
	background: #2ecc71;
  color: white;
}