Archive Page for Multiple Custom Post Types

Let’s say you have custom post types cpt_a, cpt_b, cpt_c.
You could make an archive page for cpt_a: archive-cpt_a.php.
Then after the main loop, you could make a second loop for cpt_b like this:

$cpt_b = new WP_Query( array( 'post_type' => 'cpt_b',
                    'posts_per_page' => -1 ) );
            while ( $cpt_b->have_posts() ) : $cpt_b->the_post();
                the_title();
            endwhile; wp_reset_query();

And so on…

Or you could make a custom template file with all three cpt-s like this if you need that cpt_a archive page for other purposes.