Hello, apple wath series 3 is compatible with a iPhone 6plus ?

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

If it is a new watch 3 then NO,if it has watchos5 or lower then YES it will pair;

A new apple watch 3 or 5 will NOT pair with an iphone 6 or lower,apple watch 3 and 5 have software version watchos6 on them,this requires an iphone with ios13 or higher on them.Iphones with ios13 are an iphone 6s, or higher.Iphone 6 can only update to ios12 maximum.