Do export your project, use the option "Cordova (iOS)".
Then you have a dialog that allows you to chose iOS build.
If you leave at "Cordova", you'll be able to attempt to use the Cordova project in a build service like Phonegap Build - https://build.phonegap.com/ - which has its own documentation - http://docs.phonegap.com/phonegap-build/ - on how to be used.
Otherwise, you can select "Xcode Project" that will provide you with Xcode project to open and use on a mac in Xcode like for any other application.
The provided project should compile as is. And anyway, if you were to encounter any issue doing so, you can visit the bug database and make sure your bug was not already reported.
If it was, you may add informations as comment to the ticket.
Otherwise create a new report and fill all the informations required to allow our devs to investigate in order to fix in the next releases of Construct 3.
If you search for the term "Build IOS" in the Construct 3 forum you actually get returns :