How do I add 3rd level of dropdowns to wp_nav_menu array?

how do I get 3rd level to show?

<?php
                        wp_nav_menu(
                            array(
                                'theme_location'  => 'primary',
                                'container_class' => 'collapse navbar-collapse',
                                'container_id'    => 'navbarNavDropdown',
                                'menu_class'      => 'navbar-nav ml-auto',
                                'fallback_cb'     => '',
                                'menu_id'         => 'main-menu',
                                'depth'           => 2,
                                'walker'          => new Understrap_WP_Bootstrap_Navwalker(),
                            )
                        );
                    ?>