Apple Watch Series 3 Compatibility

I would be very careful about buying a used Apple Watch and think even harder about a Series 3. They're VERY difficult to update. It is no longer eligible for the latest WatchOS. And if you can't see the watch in person to make sure it has had Activation Lock removed, you could wind up being one of the many who come each day asking how to use a used watch they bought which is Activation Locked. And the answer is they can't. So many come here to find they threw their money away on a useless watch, they can never use.

If you do decide to purchase a used Series 3, make absolutely certain this has been done on the watch or you'll be very disappointed --> What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support

In the link is a section for people who are looking to buy a used watch.

it should, compatibility is usually the other way around as in newer watches would not work with older phones, that being said there are many posts about issues getting older watches to connect, and apple watch 3g will not be receiving any updates of watchOS so it's left behind and likely many app developers will stop supporting the older watchOS version, so if it was me then I would not spend too much on a second hand 3g