Creating template with h3 block (block editor)

Just figured out the solution: use 'level' => 3 as parameter.

register_post_type('custom-post', array(
  // [...]
  'template' => array(
    array(
      'core/heading',
      array(
        'level' => 3
        'placeholder' => 'this is a H3'
      )
    )
  )
));