Does Apple Watch series 3 work with iPhone 6 plus

The current new Series 3 requires iOS 14, and the iPhone 6 plus will only upgrade to iOS 12

You need an iPhone 6s, SE, 7, 8, X, Xs, 11 or 12

Only if the watch is running an OS below 6.0.

(191594)

It will if the watch has WatchOS5 or lower.