Apple Watch Series 3 are constantly restarting!

Erasing the watch and setting it up as a new device should download the latest watchOS for your Series 3.

Unpair and erase your Apple Watch - Apple Support

Given the age of your watch, the battery might have to be replaced again. I had to get the battery replaced for my old Series 5 twice since I bought it 5 years ago.

Has the battery ever been replaced by Apple?

If not, and given the age of the watch, the battery is at the end of the usable lifetime and causing this issue.

However, since the Apple Watch Series 3 is a vintage Apple product, you may not be able to get the battery replaced:

Obtaining service for your Apple product after an expired warranty - Apple Support

Apple Service and Repair for Apple Watch - Apple Support

these forums are user helping other users like yourself, it's sort of going to a

physical apple store and asking other customers their advice

This is a user helping users with tech questions forum, we are not apple and apple do not read your posts here

non of us can give you a new watchOS version

try

Get Support (apple.com)

And or

Contact - Official Apple Support

And or

https://www.apple.com/contact/

Or

Genius Bar Reservation and Apple Support Options - Apple

or give them feedback

Product Feedback - Apple

Did Apple replace the battery of your Series 3?

I'm afraid, since the model is listed as Vintage, watchOS 8.8.1 is the latest version.

Obtaining service for your Apple product after an expired warranty - Apple Support

If the watch reboots itself although the battery health is above 80%, there might be other hardware issues involved.

This article lists the latest watchOS 8 versions:

About watchOS 8 Updates - Apple Support

Apple released watchOS 8.8.2 today

Still same issue happen even uploaded watchOS 8.2.2

Anyone else experiencing crashes/pairing … - Apple Community

I still think that this is hardware related and not a software issue.

Hello Ingo2711 san

I appreciate your reply.

My watch was not similar problem until updated watch OS 8.8.2.

That is why I suspect software problems

As expected, it turns out that there were actually people, not just me, who were troubled by the same symptoms.

Anyone else experiencing crashes/pairing … - Apple Community