Am I going to be charged for cancelling a 3-day free trial from the App Store?

As long as you cancel the trial subscription 24 hours before the end of the trial, it will not convert to a paid subscription.

Managing auto-renewing subscriptions -

If you want to cancel a subscription from Apple - Apple Support

You might need to scroll down to find the Cancel Subscription button.

Scroll further down the link for instructions for non-Apple devices.

Things about subscriptions;

⁃ Deleting an app does not cancel the subscription.

⁃ A trial subscription must be canceled more than 24 hours before the renewal date or it may be renewed.

⁃ If you have used a free trial subscription previously for an app, you will usually be charged immediately for a paid subscription if you download the free trial again.

⁃ Many subscriptions are set to renew automatically until you cancel them.

⁃ If you see multiple subscription offers, as long as you don’t select one, the subscription shouldn’t renew.

⁃ If you don’t see Cancel Subscription, the subscription is canceled and won't renew.

⁃ An expiration date means the current subscription has been canceled but is available to use until that date.

⁃ Use Resubscribe/Renew if you change your mind about canceling the subscription.

About canceling subscriptions in Germany, Israel, South Africa, or Türkiye - Apple Support

If all else fails, check out Apple Subscription Support for articles and contact options;

https://getsupport.apple.com/?PGF=PGF63006&category_id=SC0060&symptom_id=23208

M1 Mac mini, macOS 15.4.1;  Watch, series 10 GPS-only, watchOS 11.4; iPhone 16, iOS 18.4.1; TV 4th Gen, tvOS 18.4, HomePods, audioOS 18.4 and Airport Express w/ Airplay 2