The three-year subscription is a one-time purchase that does NOT renew automatically. Your subscription will simply expire at the end of your three-year term and no action is needed on your end to cancel. You can always choose to reactivate your Pro subscription at alltrails.com/pro after it expires.
NOTE: If you purchased a three-year subscription via the iPhone or Android app, this will show up in your purchase history and will NOT be listed as a subscription. Apple and Google Play consider this a one-time purchase, therefore it is not treated as a subscription on these platforms.