Hi Abbey_G,
Thanks for contacting Apple Support Communities!
We understand that you'd like to take advantage of three months free of Apple Fitness+.
Looking at the Terms & Conditions for the Fitness+ Promo, there is a stipulation to claim the promotion within 3 months of device activation.
• Offer must be claimed in the Fitness app on your iPhone within 3 months after first activating your Eligible Device (or 3 months following October 24, 2022 if your device was activated between September 7, 2022 and October 24, 2022. To see the offer appear, you will need to sign in with your Apple ID on your Eligible Device.
Fitness Plus Promo TandC's from this page: Offers - Apple
If you need additional assistance in redeeming Apple Fitness +, we'd suggest that you contact Apple directly. You can find their contact details here: Get Support
Take care!