Cancel free 3 day trial on my iPad

Did you buy a subscription or In-App purchase?

If you want to cancel a subscription from Apple - Apple Support

But not all In-App purchases can be refunded by Apple. In that case you should contact the developer of your app about that, as explained in this article:

How to contact an app developer - Apple Support

Follow the steps mentioned in this article to request a refund from Apple:

Request a refund for apps or content that you bought from Apple - Apple Support

If you can not find your HP subscription in Settings/Apple ID/Your Name/Subscriptions, did you also have a look at the HP smart printer Support page for more information?