Will an Apple watch Series 3 pair with an IPhone 6 plus?

If the Apple Watch Series 3 is new, and already has watchOS 6 on it, it will not pair with an iPhone 6 Plus. That device cannot update to iOS 13, which is required to pair with a watch running watchOS6. You would need an iPhone 6s or higher.

Yes. From the spec sheet:

Apple Watch Series 5 and Apple Watch Series 3 require an iPhone 6s or later with iOS 13 or later.

Features are subject to change. Some features, applications, and services may not be available in all regions or all languages. View complete list.

The current shipping Apple Watch Series 3 and 5 models require an iPhone 6s, SE, 7 or newer running iOS 13.

The iPhone 6 Plus Cannot upgrade to iOS 13.

If you already purchased the 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