There's more than one smartwatch out there (despite Fitbit's best efforts to buy up all the competition), and more than one that works with your iPhone. Well, "works" is a relative term. The Samsung Gear S3 does indeed work with your iPhone, but what it can do is limited. This isn't the fault of the Gear S3 though.
In this video, MrMobile looks at what you can do with your iPhone-paired Gear S3, and what you ought to be able to do, but simply can't. Give it a watch (pun intended) to see exactly how frustrated you should expect to be when dealing with this situation, and why you should probably look into an Apple Watch instead.