[Waldbaer]

.... Offering software that works on all three would make it a very appealing choice for those groups so that there is less complication with sharing songs/setlists, etc. 

I cannot stress that enough. Users (including myself) do not care about platform borders (why should they) and cross platforming is a major reason for any application to consider buying for me. Especially for apps where I'm probably not the only one to use them, of course. And looking from now, you would even be the only sheet-music-app featuring all tablet OSs, as far as I know... not to mention the benefits, that MSP already has. 

But of course this is a major work for you, so take your time!