Reply To: How can I do custom template for single posts?

Try this:
Rename the file you created “special-post-template.php” to “singular.php”
Then add this to the customizer -> additional css:

#main {
    display: flex;
}

#main #content {
    order: 2;
}

#main #sidebar-left {
    order: 1;
}

#main #sidebar-right {
    order: 3;
}

Caution: Just in case you don’t know. If you’re not using a child theme your changes will be deleted when the theme is updated.