Series 3 update

If your Apple Watch Series 3 won't pair with your iPhone and is saying that it needs an update, there could be a few different issues causing the problem. Here are a few things you can try to troubleshoot the issue:

1. Make sure both devices are charged: As you mentioned, it's important to make sure both your iPhone and your Apple Watch are charged when performing updates. If either device has a low battery, it could cause problems during the update process.
2. Check for updates on your iPhone: Make sure that your iPhone is running the latest version of iOS. You can check for updates by going to the Settings app, tapping "General," and then tapping "Software Update." If there is an update available, download and install it before trying to update your Apple Watch again.
3. Restart your iPhone and your Apple Watch: Sometimes, simply restarting your devices can resolve issues like this. To restart your iPhone, press and hold the power button until you see the "slide to power off" slider. Then, slide the power off slider to the right to turn off your iPhone. Once your iPhone is off, press and hold the power button again to turn it back on. To restart your Apple Watch, press and hold the side button until you see the power off slider. Then, slide the power off slider to the right to turn off your Apple Watch. Once your Apple Watch is off, press and hold the side button again to turn it back on.
4. Unpair and re-pair your devices: If the above steps don't help, you may want to try unpairing your iPhone and your Apple Watch and then re-pairing them. To do this, go to the Watch app on your iPhone, tap "My Watch," and then tap "Unpair Apple Watch." Follow the prompts to complete the unpairing process, and then follow the steps to pair your devices again.

I hope one of these steps helps resolve the issue you're experiencing. If you have any other questions, please don't hesitate to ask.