Apple Watch series 3 features without iOS 13

You specified Series 3 in the title and Series 2 in the body of the message.

The current shipping Apple Watch Series 3 and 5 models require an iPhone 6s, SE, 7 or newer running iOS 13.  It will Not pair with the iPhone 6 Plus or older.

If you are talking about a used Apple Watch, then it Must Not have been upgraded to watchOS 6. It must still be running watchOS 5.

If buying used...

What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else

https://support.apple.com/en-us/HT208855

Make sure you can contact the seller after the sale so they can correct any thing they missed, or give your money back.

It is not uncommon for one-off used Apple Watch sales being stolen Apple Watches that are impossible to unlock, and the seller ghosts the account after getting their money.

atjfromdubai wrote:

Yeah so a doubt......some authorized Apple sellers are selling the Apple Watch series 3 which have watchOS 5 preinstalled and not updated. So if I buy one of those will I be able to pair it with my iPhone 6 Plus which is running iOS 12.4.4 ?

Yes. But it must have watchOS 5 on it.

Also if I’m able to pair it, then will it ask me to update the watch to “watchOS 6” and after that will it hit me with the “you need iOS 13” kind of messages?

if so, then how can I avoid this?

No. watchOS 6 requires iOS 13 and your iPhone will not upgrade to iOS 13.

If at some future date you get a newer iPhone, then you should be able to transfer your Apple Watch to the new iPhone and upgrade the bits

https://support.apple.com/en-us/HT205189

Correct.