Am I eligible for the Apple Fitness+ 3 months free promotion after buying my Apple Watch Series 9?

If your account is eligible it will show you the offer when you go into the Apple Fitness app but it’s likely not available since you have already had a trial (that one is generally for new subscribers). Sometimes it may offer you a different option such as only 1 month versus 3 but again, this will show if your account is eligible.

Apple Fitness+ - Apple