I have a friend that has a Series 3 watch, and an iPhone 11. Her exercise ring does not register, but the other two do. Is this just a compatibility issue? Odd that two rings work.
Start a workout on Apple Watch
Suggestion #1: Is she starting a workout? I am assuming yes since that’s obvious, but I also have a series 3 and an iPhone 11 and mine works fine.
The Apple Watch should recognize exercise by her heart rate increasing so maybe she accidentally turned some settings off on her phone?
On her iPhone, make sure that Location Services is turned on. Go to Settings > Privacy > Location Services.
Also on her iPhone, check that Motion Calibration & Distance is enabled. Settings > Privacy > System Services and turn on Motion Calibration & Distance.
Reset your Apple Watch
Suggestion #2: I had an iPhone 10 and then 12 with a Series 3 watch and never had any issues. Tell her to try rebooting the watch, then if that doesn’t work unpair/re-pair. If all else fails, tell her to contact Apple. She should turn her phone off entirely, then the Apple Watch, let them both sit for a minute or so then turn them both back on. Hopefully that fixes it!
Suggestion #3: I have the same devices and have had the same problem. I have to reset my watch once in a while and fixes the issue.