Is an Apple Watch series 3 compatible with an iPhone 6?

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 or older.

If you have already purchased your Apple Watch, then...

Apple Holiday Return Policy (Purchase between: 15-Nov-2019 thru 25-Dec-2019, Return by 8-Jan-2020)

All purchases made before November 15, 2019 and after December 25, 2019, are subject to the Standard 14-day Return Policy.

https://www.apple.com/shop/help/returns_refund

No. “Apple Watch Series 5 and Apple Watch Series 3 require an iPhone 6s or later with iOS 13 or later.“

Only if the watch is running an OS version earlier than 6.0; otherwise, it won't pair with the iPhone.

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