Is Apple Watch series 3 compatible with iPhone 6 plus?

Yes. I've had my iPhone 6 Plus for some time. I currently own an Apple Watch series 4 and it is compatible with my iPhone 6 Plus. But, more importantly to you: Before I got my Apple Watch series 4, I had an Apple Watch series 3 and it too was compatible with my iPhone 6 Plus.

BobHarris is correct. I was wrong. I am sorry.

From Apple:

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

My old situation is not the same with new Apple Watch Series 3.

Thank you BobHarris.

The newly sold Apple Watch Series 3 models will start shipping with watchOS 6, which will not work with the iPhone 6, 6 Plus and earlier iPhones.