Hey! Not, it's not mandatory! You could make a Linux build :) 

If that doesn't work for you, you can just submit a Windows version, but then I can't guarantee we'll be able to play it (might not work well with Wine).

However, building a project doesn't count in the 3 hours, so you can spend time on it after the development. Or you can try building some project beforehand to learn a new thing. That said, you don't have to, only if you wish to try it out :)