App Store Queries

If an app is compatible with all three devices then you only need to pay for it once, it will then show in the Purchased tab in the App Store apps on your other devices for redownloading for free.

But some apps are iPad-only, and some are iPhone-only (e.g. those which require full-phone capability), and some developers write separate versions of their app for the iPad and for the iPhone/iPod Touch (in which case the two apps would be treated as separate purchases; though you should be able to use the iPhone/iPod version on an iPad, it just won't make best use of the larger screen).

An app's requirements/compatibility (including minimum iOS version) should be shown on its description page in the store.