Edit in JSFiddle

<div class="flex-container">
  <div>1</div>
  <div>2</div>
  <div>3</div>  
</div>

<div class="flex-container stretch">
  <div>1</div>
  <div>2</div>
  <div>3</div>  
</div>
.flex-container {
  display: flex;
  flex-flow: row wrap;
  background: #ddd;
  justify-content: center;
  align-items:center;
  height:200px;
}

.stretch {
  background: #cccccc;
  justify-content: center;
  align-items:stretch;
}

.flex-container>div {
  background-color: #ffffff;
  width: 100px;
  margin: 10px;
  text-align: center;
  line-height: 75px;
  font-size: 30px;
  border: 2px solid #00a9d4;
  font-family: Calibri, sans-serif;
  border-radius: .25rem;
}