Good day,
is it possible with flexbox to force a break after the second element and then just put it underneath? 
.flex {
  display:flex;
  flex-direction: row;
  width: 100%;
}
.item {
  background-color: red;
  width: 33.333%;
  min-height: 400px;
  margin: 10px;
}<div class="flex">
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
  <div class="item"></div>
</div> 
     
    