Merge menus for mobile menu
As I'm also looking for a solution to this issue I've come a bit further and maybe it will get you in the right direction.

To load a menu in Drupal 8 you can put this in a theme_preprocess_page in your .theme-file.

  $menu_tree = \Drupal::menuTree();
  $menu_name = 'main';

  // Build the typical default set of menu tree parameters.
  $parameters = $menu_tree-&gt;getCurrentRouteMenuTreeParameters($menu_name);

  // Load the tree based on this set of parameters.
  $tree = $menu_tree-&gt;load($menu_name, $parameters);

  // Transform the tree using the manipulators you want.
  $manipulators = array(
    // Only show links that are accessible for the current user.
    array('callable' =&gt; 'menu.default_tree_manipulators:checkAccess'),
    // Use the default sorting of menu links.
    array('callable' =&gt; 'menu.default_tree_manipulators:generateIndexAndSort'),
  );
  $tree = $menu_tree-&gt;transform($tree, $manipulators);

  // Finally, build a renderable array from the transformed tree.
  $menu = $menu_tree-&gt;build($tree);
  $menu_html = \Drupal::service('renderer')-&gt;renderPlain($menu);


I have not yet worked out how to merge different menus into one big menu but I will let you know if I come up with a solution.
You can find out more about the code above at Drupal 8.2.x » menu.api.php which is where I found it.

Ok, I now have found the solution which I found in the module Responsive and off-canvas menu

  $menus = array("main-menu","footer-menu","utility-menu");

  $combined_tree = array();
  $menu_tree = \Drupal::menuTree();
  $parameters = $menu_tree-&gt;getCurrentRouteMenuTreeParameters(trim($menus[0]));
  $manipulators = array(
    // Show links to nodes that are accessible for the current user.
    array('callable' =&gt; 'menu.default_tree_manipulators:checkNodeAccess'),
    // Only show links that are accessible for the current user.
    array('callable' =&gt; 'menu.default_tree_manipulators:checkAccess'),
    // Use the default sorting of menu links.
    array('callable' =&gt; 'menu.default_tree_manipulators:generateIndexAndSort'),
  );
  // Force the entire tree to be build by setting expandParents to an
  // empty array.
  $parameters-&gt;expandedParents = array();
  // Iterate over the menus and merge them together.
  foreach($menus as $menu_name) {
    $tree_items = $menu_tree-&gt;load(trim($menu_name), $parameters);
    $tree_manipulated = $menu_tree-&gt;transform($tree_items, $manipulators);
    $combined_tree = array_merge($combined_tree, $tree_manipulated);
  }

  $menu = $menu_tree-&gt;build($combined_tree);

  $vars['mobile_menu'] = array(
    '#prefix' =&gt; '&lt;div class="off-canvas-wrapper"&gt;&lt;div id="off-canvas"&gt;',
    '#suffix' =&gt; '&lt;/div&gt;&lt;/div&gt;',
    '#markup' =&gt; \Drupal::service("renderer")-&gt;renderRoot($menu),
  );


If you put this in your theme_preprocess_page in your .theme-file you could then in your page.html.twig print the twig variable {{ mobile_menu }}.