volucris1

<div class="parent">
  <div class="child">1</div>
  <div class="child">2</div>
  <div class="child">3</div>
  <div class="child">4</div>
  <div class="child">5</div>
</div>

.parent {
  display: flex;
  flex-direction: row;
  flex-wrap: wrap;
  flex-basis: auto;
  
  width: 100%;
  height: 250px;
  background-color: indigo;
}

.child {
  width: 33.33333%;
  height: 100px;
  background-color: pink;
  border: 1px solid indigo;
  box-sizing: border-box;
}

.child:nth-child(3n+1):nth-last-child(2),
.child:nth-child(3n+1):nth-last-child(2) + div {
  width: 50%;
}

.child:nth-child(3n+1):last-child {
  width: 100%;
}