Apple Music was cancelled 3 months ago and I’ve still been charged for 3 months?

Are you sure it was for the “Apple Music Membership”?

check your purchase history to see what the charges say they are for

View, change, or cancel your subscriptions - Apple Support to confirm you had cancelled and while your in the subscriptions see if there’s any apps that might be charging renewal fees.

go to this link and follow instructions - View, change, or cancel your subscriptions - Apple Support

Go here for a refund request - iTunes Store - Contact Us - Official Apple Support