Show comments from multiple post IDs in comment template

you can get comments from each post by its id with

$comments253 = get_comments('post_id=253');
$comments724 = get_comments('post_id=724');
$comments798 = get_comments('post_id=798');

then merge ( array-merge ) and sort the array by date ( comment->comment_date being the key to the date value) if you want.
then just

 foreach($comments as $comment) :
      echo($comment->comment_author . '<br />' . $comment->comment_content);
 endforeach;

This is all very manual, and you might want to automatize the process, but that’s probably an different matter.