The Apple Watch Series 3 is already running into a few compatibility issues. For the latest watchOS updates, you have to fully wipe the Series 3 to install them, removing any collected data it might have. That’s because the Series 3 has such small storage (8GB) that even the latest operating system updates struggle to work around it. We don’t know the specifics for watchOS 9 yet, but it’s likely to be even more demanding. The same is true of the apps used in watchOS, which have been developed and updated for the most recent Series 7, not the Series 3.