I have an iphone 6 plus and want to pair an apple watch series 3; how?

The iPhone 6 plus will not upgrade past iOS 12, but a new Apple Watch series 3 needs iOS 14.

iOS 14 requires a minimum of an iPhone 6s, an SE, 7, 8, X, XR 11 12

It will pair with a watch 3 if the watch is running WatchOS5 or lower.