Adding two submenus to a menu

Hope you’ll find this well , I’m adding the code below. This is one of the way of doing it, you can do things in other ways also.

add_action('admin_menu','create_admin_menu');
function create_admin_menu()
{
    add_menu_page   (
                        'Page Title',
                        'Top level menu Title',
                        'manage_options',
                        'your_unique_slug',
                        'your_magic_function'
                    );

    function your_magic_function()
    {
        include_once('first.php');
    }

    add_submenu_page    (
                            'your_unique_slug',
                            '',
                            '',
                            'manage_options',
                            'your_unique_slug',
                            ''
                        );

    add_submenu_page    (
                            'your_unique_slug',
                            'Submenu2',
                            'Submenu2',
                            'manage_options',
                            'your_unique_submenu_slug1',
                            'your_magic_function2'
                        );


    function your_magic_function2()
    {
        include_once('second.php');
    }

    add_submenu_page    (
                            'your_unique_slug',
                            'Page Title2',
                            'Submenu Title2',
                            'manage_options',
                            'your_unique_submenu_slug2',
                            'your_magic_function3'
                        );

    function your_magic_function3()
    {
        include_once('three.php');
    }
}

first.php

<h1>First</h1>

second.php

<h2>Second</h2>

three.php

<h3>Third</h3>