The ding only makes the noise when:
You physically connect it to the charger.
The Device has unexpectedly restarted, watchOS has finished loading, and after the OS finishes booting, it realizes it is connected to a charger (So it re-plays the Sound Effect).
Dare I say your Apple Watch Series 3 is massively out of date on watchOS updates.
About watchOS 8 Updates - Apple Support
(watchOS 8.7.1 is the maximum for Series 3 (as it's not compatible with watchOS 9 that just came out)
Should probably figure out how to update watchOS to the latest to stop your watch from Restarting on the Charger (assuming software crashes of WatchOS vs a hardware issue). If it won't update, maybe you're still using an old iPhone hardware + old iOS as well that doesn't support watchOS 8.7.1 paired Apple Watches.