Can I contact an Apple Watch owner? If o do t know who it is? I found the Apple Watch three months ago

No you can't. And you're not speaking with Apple here on this user to user only forum. But if you were, Apple doesn't get involved in lost or stolen devices.

I do want to thank you for wanting to reunite the watch with its rightful owner (am wondering why you're asking 3 months after finding it though).

Turn it over to the local authorities where you live is the thing to do.

After 3 months, the owner has likely given up on finding the watch and likely already replaced it. Keep it charged for a little while, but ultimately, you should turn it over to the local authorities.

It is really good of you to want to reunite the watch with its owner. Really, it is.

My Apple Watch disappeared late March. It was an SE 2nd gen black one. I’ve been trying to track mine but it’s dead and I can’t. AT&T said there was nothing I could do

If it is not showing in Find My then it is lost.

It had a new number and I can’t see the last place it was anymore

It is lost.