This uses classes from /wp-includes/css/dist/block-library/style.min.css?ver=5.4.1 .
<div class="contain">
<div class="wp-block-columns alignwide has-3-columns">
<div class="wp-block-column">...</div>
<div class="wp-block-column">...</div>
<div class="wp-block-column">...</div>
</div>
</div>