After over a decade of bringing you the latest Diablo news and builds, DiabloFans will be shutting down on March 21st, 2019. On this date, we will archive the site in an effort to preserve the history. Thanks for your patronage throughout the years.
I'm using a digital account, but it is now active and I can visit the auction house. I've tried using my friends' emails and I've tried using one of their battle tags. It always seems to tell me "Error 3"
Try connecting to your Battle.net Account Management page and changing the account's password. That will force a refresh of the account data and ought to remove any remaining trial restrictions which may be involved with the error you're seeing.