Hey there! Nice to meet you!
Unfortunately, it is not possible to redeem the free digital subscription without an active payment method, as the automatic renewal is enabled by default.
You can try asking your parents if they can lend you a payment method and add it on account.microsoft.com > Payment Options.
Once the payment method is added and the 3-month subscription is redeemed, you can remove the payment method to avoid automatic renewal upon expiration by cancelling the recurring billing first on account.microsoft.com > Subscriptions > turn off recurring billing.
Then, remove the payment method under Payment Options.
Thank you for your kindness and understanding! Please feel free to let me know if you need anything else.
Best regards,
Darhyl