Hello I have an Apple Watch Series 3 and when I try setting it up it tell me to update date it once the update finishes it says preparing and it never prepares

Try the steps here to troubleshoot not being able to update: Update your Apple Watch - Apple Support

Need more help with updating?

Try these steps:

1.    Make sure that your Apple Watch is connecting properly to its charger. 

2.    Restart your Apple Watch.

3.    Restart your paired iPhone.

4.    Try again to update your Apple Watch.

If the update won't start:

1.    Open the Apple Watch app on your iPhone.

2.    Tap General > Software Update.

3.    Delete the update file.

4.    Try again to update your Apple Watch.

If you still have problems Contact Apple Support.

Click here > Contact - Official Apple Support

or phone numbers 

Click here > Contact Apple for support and service - Apple Support