Actually, C# is perhaps the most cross-mobile option thanks to the guys over at Ximian who created M0n0Touch and M0n0Droid. Unlike the various phone gap like things, they actually compile down to native code (objective-c or davlik) and while also reflecting the native UI and device APIs. So you are actually creating 1st class iOS or Droid apps, just not in objective-c or java. It also means that the UI/device specific bits of your apps will not be directly reusable.
Now, your server-side C# app where most of the magic happens anyway don't care what the client is so long as it can consume your web services.
All that said, as a student there isn't any reason you should not be learning all three stacks.