Its a bit of a mockery to talk about something being 'cross platform' when you're talking about running on three different versions of Windows. It might make sense to focus on Windows as a Desktop OS, but the xbox doesn't have equivalent market share, and WP7 isn't doing very well at all by comparison with the Android and iphone platforms.
For extra points, abstract your game logic, port the rendering, input, etc to Unity, to have it even more cross platform:
PC, mac, xbox indie games and arcade, wii, ps3, wp7, android, and iphone.