Manage your Apple Music membership - Apple Support -"Learn how to change from an Individual membership to a Family membership, turn off automatic renewal, or join Apple Music later." Turn off automatic renewal and your membership will finish at the end of the current subscription period. --> https://support.apple.com/HT204939
When you sign up it mentions a 3 month free trial. Did you not expect to have to pay after 3 months? All subscriptions are automatically renewing until you disable them.
Apple does not normally issue refunds for subscriptions. You can try but you will have to make a special request:
iTunes [Store] Support - Use this form for assistance with account and billing, code redemption, or purchase questions -->
https://www.apple.com/emea/support/itunes/contact.html
Contact Apple for support and service - "See a list of Apple phone numbers around the world." - https://support.apple.com/HT201232