The easier way will be making the following changes in single.php file:-
Copy the following:
<?php if ( ot_get_option( 'related-posts' ) != '1' ) { get_template_part('inc/related-posts'); } ?>
And replace with the following:
<?php if ( ot_get_option( 'related-posts' ) != '1' ) { get_template_part('inc/related-posts'); } ?>
<?php if ( ot_get_option( 'related-posts' ) != '1' ) { get_template_part('inc/related-posts'); } ?>
Basically, it calls the get related post function twice, and it’s possible that duplicate post will occur between top 3 related post and bottom 3 related post. However, that will do the trick.