Edit in JSFiddle

<div class="card">
  <div></div>
  <div></div>
  <div></div>
  <div></div>
  <div></div>
  <div></div>
</div>

.card {
  width:320px;
}
.card div {
  width:100px;
  height:100px;
  background:blue;
  float:left;
  margin:0 10px 10px 0;
}
.card div:nth-child(2n+2) {
  background:yellow;
}
.card div:nth-child(3n+3) {
  margin-right:0;
}