This doesn’t really depend on Ionic 3, but on Cordova/Capacitor and/or the plugins you use.
Ionic 3, as framework supports iOS 8+.
So, if you develop an Ionic 3 app and deploy it as a web app, it will work fine on iOS 8+.
But if you want to deploy it to the App Store, then you’ll have to use Cordova or Capacitor to deploy the app, in that case:
If you use Capacitor, it requires iOS 11+.
If you use Cordova:
cordova-ios 4.x supports iOS 9+.
cordova-ios 5.x supports iOS 10+.
If you use cordova-plugin-ionic-webview, 4.x, it requires iOS 11+.
If you use cordova-plugin-ionic-webview, 2.x, it requires iOS 10+.
All those requirements are independent from the Ionic version you use.