Hey there,
I’m not seeing a Diablo 3 purchase on the Battle.net account that you’re posting from. You will see an active game “license” under your Games & Subscriptions, however that is just because you need a license to be able to play even the free version of the game.
If you did have a purchase it would show up under Transaction History
You may want to try to remember if you used any previous or alternate email addresses to log in to Battle.net in the past, and try logging in or resetting the password if you can recall it. If you have an email with the order # of the digital purchase you can also contact support and they can assist with locating the account that purchase was made on.