it should format as expected
<div
  class="bundle-product"
  data-variantId="{{ variant_id }}"
  {% for i in (1..3) %}
    {% assign plan_id = i | minus: 1 %}
    data-sellingId-{{ i }}="{{ product.selling_plan_groups[0].selling_plans[plan_id].id }}"
  {% endfor %}
></div>