<div class="container">
  <div class="item">item content 1</div>
  <div class="item">item content 2</div>
  <div class="item">item content 3</div>
</div>

.container
{
    display: flex;
    flex-wrap: wrap;
}
.item
{
  flex-basis: 50%;
}
.item:nth-child(2n-1)
{
   background-color: red;
}
.item:nth-child(2n-2)
{
   background-color: yellow;
}
div.item:last-child
{
   flex-basis: 100%;
   background-color: green;
}