Why is my update taking forever ?

Three hours is too long. I have had some that took up to an hour or more but not that long. Once when it looked like it would never update I reset my watch by pressing in the crown and side button until I saw the Apple logo. Make sure that your iPhone is updated first, then try again.

Update your Apple Watch - Apple Support

"It could take from several minutes to an hour for the update to complete. You might want to update overnight or wait until you have time. "