<div class="row row-cols-1 row-cols-md-3">
<?php for ($i=0; $i<9; $i++) { ?>
  <div class="col mb-3">
    <div class="card card-block">
      <img src="..." class="card-img-top" alt="...">
      <div class="card-body">
        <h5 class="card-title">Card title</h5>
        <p class="card-text">This is a longer card with supporting text below as a natural lead-in to additional content. This content is a little bit longer.</p>
      </div>
    </div>
  </div>

<?php } ?>