I have a Windows 7 PC, and I think it has something to do with the F3 key. I don't have the Fn (function) key, I tried pressing F3 with all the keys I have, and have had no luck.
You only need to press the function key (fn) if you have little functions assigned to the keys themselves:
I have a brightness up on my F3 key, but I need to press fn+F3 to use it, but this does not affect my controls when playing Minecraft or any other action that requires the use of F3.
When in doubt, never forget there's a wiki.
This link may help as well.