Jetpack Related Posts: show random post if no related post is found

Hi guys!

I am posting here but I’m not sure if this is the right place on the forum.

I have a doubt about Jetpack’s Related Posts.

Does anyone know how I can show a random post if no related post was found?

For example: I always show 3 posts on my related posts.

If, for any given post, only 2 related posts are found, how can I display a random 3rd post, just to fill in the gap and get the number of 3 posts?

I tried to google this but my English is so bad that I couldn’t find an answer.

I hope you can understand me ??

Thank you guys!