<main class="inline-block-center">
  <div>
    This is a block-level centered in a row. 
    The white container will stretch as much as needed.
  </div>
  <div>
    This is a block-level centered in a row. 
  </div>
  <div>
    This is a block-level centered in a row.
  </div>
</main>

<main class="flex-center">
  <div>
    This is a block-level centered in a row with Flexbox.
  </div>
  <div>
   This is a block-level centered in a row with Flexbox.
  </div>
  <div>
    This is a block-level centered in a row with Flexbox.
  </div>
</main>

body {
  background: #40BDB7;
  font-size: 80%;
}

main {
  background: white;
  margin: 20px 0;
  padding: 10px;
}

main div {
  background: black;
  color: white;
  padding: 15px;
  max-width: 125px;
  margin: 5px;
  border-radius: 5px;
}

.inline-block-center {
  text-align: center;
}
.inline-block-center div {
  display: inline-block;
  text-align: left;
}

.flex-center {
  display: flex;
  justify-content: center;
}