When I use F3 or even Fn+F3, the Exposé screen comes up, not the Minecraft Coordinates in the corner. I would recommend adding an option to change the key you need to press in the settings with the others.

Even if I turn off all the settings for Function keys, it still happens. (I do believe Apple embedded the Exposé function directly into the keyboard, so it would not be possible to change minecraft to acces the F3 key.)