The topic starter is using a child theme, so this is how he managed to change the H3 tag of the comment title:
https://wp-puzzle.com/docs/basic/change-h3-in-comments-list-and-form.html
If you are not using child theme, try this:
add_filter( 'generate_comments_title_output', function( $output, $comments_title ) {
return sprintf(
'<strong class="comments-title">you custom comment title</strong>',
esc_html( $comments_title )
);
}, 10, 2 );