There are no rules for that sort of thing, and in the wild, you can encounter all
three of those formats. None of them are wrong, unless you are part of an
organization that has adopted a particular style. Some have their own
style guides, some follow another organization's style guide.
I said earlier that there might be an evolutionary process, with 1., 2., and 3 above
following each other as the word or expression develops. But I'm really not sure of that.
bill