Apple Watch Series 3 OS

Today, watchOS 5, as watchOS 6 has not been released.

After watchOS is released, it may come with watchOS 5, only because it was already boxed sitting in inventory when it was shipped to you. Eventually newly manufactured units will be made after watchOS is available, and they will be manufactured with watchOS 6.

But you get watchOS 5, you can easily upgrade to watchOS 6 after watchOS 6 is released.

If you go to Apple.com, click through until you can click "Buy" for the Apple Watch series 3, the specs for the watch says watchOS 6

So I would say depending on where you buy your Apple Watch, it will have either watchOS 5 or 6 depending on whether it comes out of someone inventory or is newly manufactured and shipped direct to you.

watchOS 5.3.1 is what my S3 has on it and I checked to see if watchOS 6 is released but not yet.

I suspect it may be rolled out maybe into the weekend? last year iOS was ready on the same day the new phones were exhibited.

looking the series 5, i do not like the always on mode. prefer it dark unless demanded. always on eats battery too much for my tastes

According to this link it comes with Watch OS 4...........https://www.stuff.tv/features/11-things-you-need-know-about-apple-watch-series-3

watchOS 6 is scheduled for release September 19th