<ul>
	<li>aaa</li>
	<li>bbb</li>
	<li>ccc</li>
	<li>ddd</li>
	<li>eee</li>
</ul>

ul {
	padding: 0;
	display: flex;
	flex-wrap: wrap;
}
ul li {
	background: #ccc;
	padding: 20px;
	margin-bottom: 20px;
	text-align: center;
	list-style-type: none;
	flex-basis: 32%;
	box-sizing: border-box;
}
ul li:not(:nth-child(3n)) {
	margin-right: 2%;
}