Hard to tell. The original theme author set it to display 3 and that’s what it does. There could be any number of differences between web sites as to why it might work with more than 3 in one configuration but not another. What you can do is set up post rows and display 3 posts per row. This is how the home and archive pages work to display 2 posts per row.
1. Copy /inc/related-posts.php from the parent theme to the same path in your child theme.
2. Make the 3 modifications I’ve added as shown by this
<!-- ====================================================== -->
in this code in Pastebin.
Let me know how that works.