Is it possible to add 3 level menu item to admin menu?

While the main admin menus (i.e., those on the left-hand side of the screen) can only be 2 deep (A > B), nodes in the toolbar can be arbitrarily deep.

I don’t know if using the toolbar would be a suitable workaround for you, but if so, then you could do something like:

add_action ('wp_before_admin_bar_render', 'wpse_admin_toolbar_test') ;

function
admin_toolbar ()
{
    global $wp_admin_bar ;

    $args = array (
        'id' => 'wpse_admin_toolbar_test',
        'title' => 'WPSE Admin Toolbar Test',
        ) ;
    $node = $wp_admin_bar->add_node ($args) ;

    for ($i = 0 ; $i < 4 ; $i++) {
        $args = array (
            'id' => "wpse_admin_toolbar_test_item_{$i}",
            'parent' => 'wpse_admin_toolbar_test',
            'title' => "Item $i",
            ) ;
        $wp_admin_bar->add_node ($args) ;

        for ($y = 0 ; $y < 3 ; $y++) {
            $args = array (
                'id' => "wpse_admin_toolbar_test_item_{$i}_subitem_{$y}",
                'parent' => "wpse_admin_toolbar_test_item_{$i}",
                'title' => "Sub Item $y",
                ) ;
            $wp_admin_bar->add_node ($args) ;

            for ($z = 0 ; $z < 2 ; $z++) {
                $args = array (
                    'id' => "wpse_admin_toolbar_test_item_{$i}_subitem_{$y}_subitem_{$z}",
                    'parent' => "wpse_admin_toolbar_test_item_{$i}_subitem_{$y}",
                    'title' => "Sub-Sub Item $z",
                    // in the real-world, this URL would be to something that
                    // would perform the action for this node
                    'href' => admin_url (),
                    ) ;
                $wp_admin_bar->add_node ($args) ;
                }
            }
        }

    return ;
}

The above would produce the following: