After updating to watchOS 3.0 my watch dies in a few hours.

Hi

Battery life varies depending on use and other factors.

In tests, for example, Apple achieved 18 hours of battery life with the following usage: 90 time checks, 90 notifications, 45 minutes of app use and a 30-minute workout with music playback from the watch via Bluetooth.

If your watch is experiencing significantly shorter battery life than before, try the following steps:

Close all open apps on your iPhone and then restart both your iPhone and your watch:

• On your iPhone: double-click the Home button, then swipe up on each app preview to close it;
• Turn both devices off together, then restart your iPhone first;
  • Restart your iPhone, iPad, or iPod touch - Apple Support
  • Restart your Apple Watch - Apple Support

Unpair and re-pair your watch:

• The Watch app on your iPhone takes a backup of your watch automatically when unpairing via the app. When setting it up again, choose to Restore from Backup. Most data and settings will be restored, with some exceptions (eg Apple Pay cards, passcode).
• Unpair your Apple Watch and iPhone - Apple Support
• Set up your Apple Watch - Apple Support

Check that you do not have workout running or paused in the built-in Workout app.

If you have ever(not necessarily only recently) previously used any third-party fitness tracking apps such as Strava to record workouts on your watch, check on your watch to see whether there is presently a paused activity within any of those apps. If there is, end that activity from within that app on your watch (not on your iPhone).

• If in doubt, it may help to remove any such app from your watch.

Under earlier software versions, some users reported experiencing battery life issues in connection with using corporate profiles and/or Exchange Calendars. If you are using an Exchange Calendar, one or more of these steps may help:

• Archive your Exchange Calendar for all items before today.
• Remove / delete all old inbox messages relating to calendar invites (accept and decline notifications).
• Create a rule in your Outlook Mail account that moves all future accept and decline replies, immediately upon receipt, into a separate mail folder that does not sync with your iPhone and Apple Watch.
• Check your Exchange Calendar for errors.

Finally, it may help to remove all third-party apps from the Dock and/or from your watch and then monitor battery performance after adding them back, one at a time. The same applies in respect of watch face complications.

Having tried the previous steps, if your watch continues to experience significantly shorter battery life than Apple's examples suggest that you should expect, I suggest that you contact Apple Support or make a Genius Bar reservation for assistance:

• Contact - Official Apple Support
• https://twitter.com/applesupport
• http://www.apple.com/retail/geniusbar/

More information:

http://www.apple.com/watch/battery.html