Apple Watch 3 barometer
Is there a way to get barometer readings using Apple Watch 3?
Apple Watch Series 3, watchOS 6
Unless there is an app in the App Store, the closest thing I am aware of is in the "See your bearings, elevation, incline, and coordinates" section of:
Use Compass on Apple Watch (Apple Watch Series 5 only) - Apple Support
(Which only applies to the series 5 watch.)