Layout a flex box similar to a table?

header, .row {
  display: flex;  /* aligns all child elements (flex items) in a row */
}

.col {
  flex: 1;        /* distributes space on the line equally among items */
}

<section>
  <header>
    <div class="col">Column A</div>
    <div class="col">Column B</div>
    <div class="col">Column C</div>
  </header>
  <div class="row">
    <div class="col">1</div>
    <div class="col">2</div>
    <div class="col">3</div>
  </div>
</section>