$nids = array(123,456,789);
$nodes = node_load_multiple($nids);
foreach ($nodes as $node) {
  $node_view = node_view($node, 'teaser');
  $output .= drupal_render($node_view);
}
$build['nodes']['#markup'] = $output;
return $build;

$nodes = Drupal::entityManager()->getStorage('node')->loadMultiple($nids);
// Or a use the static loadMultiple method on the entity class:
$nodes = DrupalnodeEntityNode::loadMultiple($nids);

// And then you can view/build them all together:
$build = Drupal::entityManager()->viewMultiple($nodes, 'teaser');

$render_controller = Drupal::entityTypeManager->getViewBuilder($entity->getEntityTypeId());
$render_output = $render_controller->view($entity, $view_mode, $langcode);