Installing the Watch beta provile on both iphone X and A-watch series 3 then making sure to restart both worked. However I also had to take the watch out of nighstand mode to be able to unlock it while charging. Otherwise, even after a reboot, I kept getting a cannot connect error dialog. Airport mode was OFF, Wifi was ON, and the two appliances were within a foot of each other. Once these steps were followed the download began, stopping and resuming on its own twice before it completed the download (5 minutes) and moved to the preparing phase (28 minutes). Install to the watch after peparing phast took 6 minutes. When watch reboot occurred, it prompted me to enter a passcode. The boot circle completed about half way and then the watch rebooted again, completing the install.
NOTE: After the install of the beta, the watch app on the iphone x indicated that T-Mobile cellular plan was installed. The watch indicated No Cellular Plan intalled. Under cellular plan in settings, Plan showed NO SIM and status was NO CONNECTION.