post_count仅适用于包含缩略图的帖子

how to edit this condition in order to count only the posts with thumbnails?

if ( $my_query->have_posts() && ( ( $my_query->post_count ) >= 3 ) )

You have to check it in a different way;

$post_with_thumbs = 0;
if ( $my_query->have_posts() ) {
  while ( $my_query->have_posts() ): $my_query->the_post();
    if (  has_post_thumbnail()) {
            $post_with_thumbs++;
        }
  endwhile;
}
wp_reset_query();

if ($post_with_thumbs >= 3) {
    //do stuff
}

You must do the loop, because thumbnail relationships are not resolved automatically:

$postsWithThumbs = 0;

while($my_query->have_posts()){
  $my_query->the_post();

  if(has_post_thumbnail())
    $postsWithThumbs++;
}

wp_reset_postdata();

print $postsWithThumbs;

I'm not sure if this still works, but from this thread you can select posts with a thumbnail by specifying meta_key=_thumbnail_id in your original query.