Put the login – logout link in a custom nav menu by adding the following code to your functions.php file.

add_filter('wp_nav_menu_items', 'add_login_logout_link', 10, 2);
function add_login_logout_link($items, $args) {
if($args->theme_location == 'menu-3') {
ob_start();
wp_loginout('index.php');
$loginoutlink = ob_get_contents();
ob_end_clean();
$items .= '<li>'. $loginoutlink .'</li>';
}

return $items;

}