Instrument types

I'm not sure there is a direct way to do that for a piece created from MIDI, since probably those three staves were created as three separate instruments. The way you'd do it for a score created from scratch is to create a single instrument with three staves. So unless someone knows up a way to directly convert a separate instrument into a staff of another instrument, I think what probably makes the msot sense is to add two staves to the first instrument then copy the contents of the last two instruments to those new staves. Then delete the now-duplicate instruments. Normally, I'd be hesitant to recommend that since copy/paste loses a fair amount of information. But actually, most of the info lost is information that wasn't present in the MIDI file anyhow - text markings, info on stem directions, etc. So as long as you do the copy before adding that you shouldn't be any worse off than you already are.