How well does the Samsung Gear S3 watch work with iPhone?
According to what Samsung has announced to date, the Gear S3 will be able to pair with specific iOS devices:


  Compatible devices: iPhone 5, iPhone 5S, iPhone 5C, iPhone 6, iPhone 6+, iPhone 6S, iPhone 6S+, iPhone 7, iPhone 7+ and iPhone SE (iOS 9.0 and above).


Samsung will create iOS apps to take advantage of whatever the Gear S3 has to offer.  So it appears that 'compatibility with iOS' just means they'll use Bluetooth to communicate with the iPhone and also make apps for it.