How do I turn co-ordinates on in Minecraft? [duplicate]

You only need to press the function key (fn) if you have little functions assigned to the keys themselves:

For Example

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.