One of the things I like about writing fantasy is that when you create your own world, no one can tell you that you haven't done your research. That doesn't mean that you can throw logic out of the window, however (unless in your fantasy world, Logic is the name of the cat).
If you decide to have three moons orbiting your planet, think about what effect they might have on it and anything living on it, and be careful if your characters take to sea when they align. Introducing three moons simply for the sake of it should probably be avoided. When you mention something, readers expect it to be significant and you wouldn't want them to feel cheated. Most importantly, be consistent. It's all too easy to trip up, especially if you're writing a novel, and forget that you gave your new world six seasons and revert to four. You might not notice your mistake, but you can be sure it will jump out at readers straightaway. The best way to avoid trouble is to make a list of anything new you invent for your world and check your manuscript against it when it's finished.
I'm off now to find some undiscovered countries of my own. Have fun!