can i use my apple watch series 3 with mi iphone 6?

The current shipping Apple Watch Series 3 and 5 models require an iPhone 6s, SE, 7 or newer running iOS 13.  They will Not pair with the iPhone 6 or older.

Getting an older Series 3 can be an issue, as many used models are not properly prepared for resale and are still locked to the original owner, so make sure you can pair the watch before you transfer any money.

If the watch has watchos5 or lower then yes,if the watch has watchos6 then no.