To my knowledge, no programming apps for the iPad, JMRI or otherwise. No direct connection to the PR3, either, since both the PR3 and iPad are "client" USB devices.
However, there is WiThrottle, a JMRI app that works under iOS. It's an operating program only, but a good one - I use it every day on both iPhones and iPads. The nice thing about JMRI and tablets is you can also turn on the web server in JMRI and access panels from the tablet's browser. The browser access also has throttle capability.