Subscriptions automatically renew unless auto-renew is turned off at least 24 hours before the end of the current period. Your account will be charged for renewal within 24 hours prior to the end of the current period at the cost of the chosen plan.
You can test the Premium features for 7 days when you subscribe to a weekly plan. The subscription will automatically start at the end of the free trial if you don’t cancel it.
You may cancel the free trial at anytime. For a step by step guide, please follow this link: https://support.apple.com/en-us/ht202039. Obviously, you will not be charged at the end of the 3-day free trial if you cancel it.