Register a block pattern using PHP. Fits into the Theme structure of https://github.com/SayHelloGmbH/hello-roots/

three-columns.php

<!-- wp:columns {"backgroundColor":"secondary"} -->
<div class="wp-block-columns has-secondary-background-color has-background"><!-- wp:column -->
<div class="wp-block-column"><!-- wp:paragraph -->
<p>One</p>
<!-- /wp:paragraph --></div>
<!-- /wp:column -->

<!-- wp:column -->
<div class="wp-block-column"><!-- wp:paragraph -->
<p>Two</p>
<!-- /wp:paragraph --></div>
<!-- /wp:column -->

<!-- wp:column -->
<div class="wp-block-column"><!-- wp:paragraph -->
<p>Three</p>
<!-- /wp:paragraph --></div>
<!-- /wp:column --></div>
<!-- /wp:columns -->

ThreeColumns.php

<?php

namespace SayHello\Theme\Pattern;

/**
 * Manage single block pattern
 *
 * @author Say Hello GmbH <hello@sayhello.ch>
 */
class ThreeColumns
{

	public function run()
	{
		add_filter('init', [$this, 'register']);
	}

	public function register()
	{
		ob_start();
		get_template_part('partials/patterns/three-columns');
		$content = ob_get_contents();
		ob_end_clean();

		register_block_pattern(
			'sht/three-columns',
			[
				'title' => __('Simple three-column layout', 'sha'),
				'categories' => ['text'],
				'content' => $content,
			]
		);
	}
}