Can't pair watch to iphone 5 iOS 10.3.3

Sorry, but you'll need at least an iPhone 5s, to be able to pair your Apple Watch:

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To use Apple Watch Series 3 (GPS + Cellular), you need an iPhone 6 or later with the latest version of iOS. (To use Apple Watch Series 3 (GPS) or earlier models of Apple Watch, you need an iPhone 5s or later with the latest version of iOS.)

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copied from: Set up your Apple Watch - Apple Support

If the Apple Watch runs watchOS 4, then iOS 11 is needed, plus an iPhone 5s or later model,

watchOS 1-3 require iOS 10 and iPhone 5 at least

You're welcome 🙂