MIDIplay is a cheap ($0.99) MIDI file player that you need. It’s pretty simple and works with AB3. Here are the details:
An app designed for Audiobus 3 that can also be used by itself to play MIDI files using a light-weight synth. Shake the device to change synth sound.
MIDI files may be downloaded from the Internet or imported via email. Files may contain multiple tracks and channels, which can be selectively turned on/off. Sliders control tempo and pitch (can be transposed +/- 6 semi-tones).
Optimized for Audiobus 3.
MIDIplay on the app store for $0.99: