apple watch3 wont hold its charge

It would help to know some detail of what you are seeing as a problem.

If the watch is not charging properly please start with Apple’s suggestions in If your Apple Watch won't charge or it won't turn on - Apple Support

If the watch seems to charge correctly then uses battery at an excessive rate there are a few things you can try to get closer to the problem. Depending how fast it loses battery,

• Charge the watch then power-off and leave it until you think it would be well down, then power up again and check the battery state. Losing charge while off indicates a defective battery, Contact Support for more advice.
• Forced reset the watch by pressing and holding the Digital Crown and Side Button (together, at least 10 seconds) until the Apple logo appears, then release the buttons and let the watch restart. Test battery life again.
• Check and if necessary update software in your iPhone and Apple Watch. You should have iOS 12.4 and WatchOS 4.3 current today. If you need to update, do the phone first. Test again!
• Unpair and re-pair the watch to force reloading all software. First attempt restore the backup, and if that doesn’t help repeat, setting up as a new watch then test. In vanilla configuration before re-downloading any apps one at a time until you find which one is eating battery.

By this stage you have exhausted all the DIY fixes, so if it is still not behaving Contact Support and arrange to get the hardware tested, with an expectation you might be in line for battery service.