Bootstrap Views Grid Unable to specify Medium and Small device classes
I had a similar issue some months ago. It seems Bootstrap Views allows this possibility, but only in the dev release, not in the latest stable release.

In order to do what I wanted in the stable release of the module, I ended up with the following solution:

/**
 * Implements preprocess_views_view__VIEW().
 */
function scicnet_preprocess_views_view__MY_VIEW(&amp;$vars) {
  $display_id = $vars['display_id'];
  switch ($display_id) {
    case 'MY_DISPLAY':
      $vars['rows'] = str_replace('col-lg-4', 'col-lg-4 col-md-4 col-sm-12', $vars['rows']);
      break;
  }
}


Not the most elegant solution, I admit, but it does the job...