Hey Myrrhman70, welcome to Three Community.
The Add-ons available to our Pay Monthly customers come in One Day Boost, Short-term Boost and Monthly Boost variations. We do try to make it clear on both the app and browser version of My3 that Monthly Boosts are recurring and will renew each month until cancelled. I'm really sorry that's not been your experience though, and that you've been charged unexpectedly.
Have you tried cancelling the Add-on via My3? If you click 'My Account' and then 'View / manage Add-ons', can you see the active Monthly 1GB Add-on and the option to cancel it? Are you getting an error message when trying to use that?
Let us know how and we'll do all we can to help.