If you’d like to run the app on your iOS and Mac / Windows device, separate purchases are necessary. The reason for this is that each app runs on a completely different software platform. Because of this, apps for each platform have to be developed from the ground up and sold separately through their respective stores i.e. App Store, Mac App Store, and the Windows Store.
However, if you have purchased one version, you can avail of a 50% discount off the original price of other versions.