Try ShellJS. It works on all three platforms, and has been battle-tested in the build systems of several Mozilla projects (PDF.js, Firefox OS, Popcorn.js, etc).

https://github.com/arturadib/shelljs

Sweet! I was just looking for a cross platform build tool but this seems like it can be useful in many other ways.

Haven't used it myself, but it is suppsoed to work, there are a number of Windows specific things on the FAQ - https://github.com/cowboy/grunt/blob/master/docs/faq.md

I run it in Windows via msysgit without issue (need nodejs and npm). It's pretty easy to get up and running quickly.

Grunt works great on all those platforms!