According to what I understand is : 

In your taxonomy.php

Add this code :
 
     $term = get_term_by( 'slug', get_query_var('term'), get_query_var('taxonomy') );
     echo '<h2>Showing posts with topic : ' . $term->name</h2>;

This will result in showing **"Showing post with topic : Cancer"**


Now for **(all posts tagged with cancer already display here...)**

You can run a get_posts() to get this :
    
    $all_post = get_posts(array(
        'post_type' => array( 'cpt1', 'cpt2', 'cpt3', 'post' ), // all cpt slug name
        'numberposts' => -1, // number of post
        'tax_query' => array(
             array(
                 'taxonomy' => get_query_var( 'taxonomy '), // current tax name
                 'field' => 'id',
                 'terms' => $term->term_id, // current tax_id
            )
        )
    );

By looping over them we can get all post details and we can display them.