You could do all three easily though, I think. As long as what you do is for your own fun, it'll go pretty smoothly. What I see is that song lyrics and such seen to pop out of nowhere when you're idly writing, and learning instruments isn't all too difficult for most people and from there out it's just practice. Really game design sounds like the most difficult to me, especially as a career, but that's probably cause I know nothing about it.
I mean, look at the resources you have. You can start writing any time, send it out to friends for evaluation and editing, and also self-publish your book online! With the right equipment you can create your own music easily; look at all the people on YouTube (Alex Day <33). You really can do anything you want. And I say go or it. As long as it's fun!
You can also write books, make a video game based on them (or vice-versa) and write the music for the game. Or about the books/game. :>