column-span

column-span: none;

column-span: all;

<section id="default-example">
  <div class="multicol-element">
    <p>
      London. Michaelmas term lately over, and the Lord Chancellor sitting in
      Lincoln's Inn Hall.
    </p>
    <div id="example-element">Spanner?</div>
    <p>
      Implacable November weather. As much mud in the streets as if the waters
      had but newly retired from the face of the earth, and it would not be
      wonderful to meet a Megalosaurus, forty feet long or so, waddling like an
      elephantine lizard up Holborn Hill.
    </p>
  </div>
</section>

.multicol-element {
  width: 100%;
  text-align: left;
  column-count: 3;
}

.multicol-element p {
  margin: 0;
}

#example-element {
  background-color: rebeccapurple;
  padding: 10px;
  color: white;
}

/* Keyword values */
column-span: none;
column-span: all;

/* Global values */
column-span: inherit;
column-span: initial;
column-span: revert;
column-span: revert-layer;
column-span: unset;

<article>
  <h2>Header spanning all of the columns</h2>
  <p>
    The h2 should span all the columns. The rest of the text should be
    distributed among the columns.
  </p>
  <p>
    This is a bunch of text split into three columns using the CSS `columns`
    property. The text is equally distributed over the columns.
  </p>
  <p>
    This is a bunch of text split into three columns using the CSS `columns`
    property. The text is equally distributed over the columns.
  </p>
  <p>
    This is a bunch of text split into three columns using the CSS `columns`
    property. The text is equally distributed over the columns.
  </p>
  <p>
    This is a bunch of text split into three columns using the CSS `columns`
    property. The text is equally distributed over the columns.
  </p>
</article>

article {
  columns: 3;
}

h2 {
  column-span: all;
}