Apple Watch Series 3 requires an iPhone 6 or later for setup; battery gets 1 hour of talk time29
This means that if you are moving up from a Series 1 Apple Watch to a Series 3 model, you are going to need to get your hands on an iPhone 6 or higher in order to set up your new watch. That iPhone 5s you've been using will no longer cut it. You should note that this is true regardless of whether you own the cellular enabled version of the Apple Watch Series 3, or the non-cellular Series 3 model.
While that might not be earth shattering news for many, this news might be. The battery inside the Apple Watch Series 3 will provide only one hour of talk time when connected to LTE, according to Apple. If the phone is paired with an iPhone 6 or higher, talk time triples to 3 hours. Overall, Apple says that the Series 3 Watch will last 18 hours with mixed usage.
The Apple Watch Series 3 can be pre-ordered starting this Friday, September 15th. The product will be released on September 22nd.
source: Apple via CNBC