Will Apple Watch series 3 work with iPhone 6?

That depends. If the watch has not been updated to watchOS 6, yes, you can use it with an iPhone 6. If, however it has been updated to watchOS 6, you'll need a phone running iOS 13, a 6S or better.

If the watch3 has watch os5 or lower then yes it will,if it has watchos6 then no it won't because watchos6 requires a phone with at least ios 13 or higher and an iphone 6 cant update past ios 12.