Hi Cloudless sky,
Thank you for using the Apple Support Communities! We understand your Apple Watch Series 3 is not syncing properly with Apple Fitness+. We're happy to provide you with guidance.
Your Apple Watch series and iPhone model are certainly compatible with Apple Fitness+, so no, this won't be a 'you need to upgrade your devices' situation, so no worries there: Devices you can use with Apple Fitness+
You've done some great steps already! Make sure you've also covered the important details shared in these support articles next: Get the most accurate measurements using your Apple Watch, and Calibrating your Apple Watch for improved Workout and Activity accuracy.
We also recommend ensuring your iPhone and Apple Watch are both up to date with software: Update your iPhone, iPad, or iPod touch, and Update your Apple Watch.
If the issue remains, you will want to get in touch with Apple Support directly, so you can speak with a specialist who can investigate the behavior further towards a solution.
Thank you, and take care!