Cannot pair my Apple Watch 3 with iPhone 6

Translation. You need an iPhone 6s, iPhone SE (1 or 2), iPhone 7, 8, X, Xs, 11, or 12

From the bottom of the Apple Watch Series 3 web page

Apple Watch Series 3 requires an iPhone 6s or later with iOS 14 or later.

Iphone 6 can only update to ios12.

WatchOS6 requires ios13.

WatchOS7 requires ios14.

Translation;

IT WON'T PAIR.

Yup! 😀