note

*{
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

.container {
  background-color: #ccc;
  padding: 10px;
  /*height: 1500px;*/
  
  display: flex;
  flex-direction: row;
  justify-content: space-between;
  align-items: center;
}

.item {
  background-color: #f1425d;
  padding: 30px;
  margin: 30px;
  color: #fff;
  font-size: 40px;
}

.i3 {
  height: 200px;
  order: -2;
  /*
  flex-basis: 300px;
  flex-shrink: 0;*/
  flex: 0 1 300px;
 
}

.i4 {
  height: 200px;
  order: 1;
  flex: -1;
}

.i5 {
  align-self: flex-end;
  order: -1;
}





<div class="container">
  <div class="item">1</div>
  <div class="item">2</div>
  <div class="item i3">3</div>
  <div class="item i4">4</div>
  <div class="item i5">5</div>
  <div class="item">6</div>
</div>