Apple Watch series 3 compatibility with iPhone 5s

Hey there!

Unfortunately, the Apple Watch Series 3 is not compatible with iPhone 5s.

Apple Watch Series 5 and Apple Watch Series 3 require an iPhone 6s or later with iOS 13 or later.

See https://support.apple.com/kb/sp766?locale=en_AU for more information

If the watch is on WatchOS5 or lower it will pair.