The Gear S3, Gear S2 and Gear Fit2 all work on iOS now, thanks to the latest app update from Samsung.
You'll need to have an iPhone 5, iPhone 5S, iPhone 5C, iPhone 6, iPhone 6+, iPhone 6S, iPhone 6S+, iPhone 7, iPhone 7+ or iPhone SE (iOS 9.0 and above) for the update to work.
Samsung does warn however, that "the services and features of Samsung Gear devices may be limited for certain iOS devices and their applications". [Samsung]
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