I have an iPhone 5 and want to use Sleep Pulse 3

Sleep Pulse 3 Motion requires iOS 11 and watchOS 4 to run:

https://finalfrontierapps.zendesk.com/hc/en-us/articles/115003331712-Initial-Setup-For-Sleep-Pulse

Unfortunately, iPhone 5 and iPhone 5c are the only devices that don't support iOS 11, yet pair with Apple Watch. Because watchOS 4 requires being paired to a device running iOS to update and run, Sleep Pulse 3 will not be compatible. In some cases, updating your Apple Watch via a device running iOS 11 then trying to pair back to your iOS 10 device may work, though there is no guarantee on this. Sleep Pulse is capable of running on iOS 10, though it is not fully supported.