Interesting, I never saw or heard about that Slovenian salute. It really is unknown to the wider public, sadly. Perhaps also because it's a bit more difficult to make in a nice looking way due to the functioning of the finger muscles and nerves (al mam pa js zjebane prste ;D ). It isn't provoking anyone, it has no insultive meaning – great.

I've heard that Serbian trije prsti also stand for some 3 wars that the Serbs fought (though I don't know which ones exactly).

I've seen the two finger gesture used a lot by šipci in the time when Kosovo became "indepentent" as a contra to trije prsti. Don't know if it has any special meaning though. Others will probably be able to tell us a lot more. :)