When you bought 3 days or 30 days membership you agreed to register for 30 days recurring subscription. After your account will expire.
This is why you were automatically charged after your premium days expired.
To stop recurring subscription, go to your profile and click Cancel subscription.
After you purchase a 3-day trial account, you will be automatically billed for the 30-day subscription after the 3 days expire.
If you activated your account using a voucher code, then you will not have any recurring subscriptions (voucher codes are used only once).