Umm.. you really don't have to tell me about these stuff which I thought to be obvious, looking at the links in my signature.
But anyway, as the unregistered said, if a dual skill weapon gets a "3rd" skill, 2 skills of the 3 get integrated into one (with different name, but same effects) making it still a dual skill weapon on display, but effectively having 3 skills. And as it was also mentioned & which I was trying to point out in my previous post as well, I highly doubt they will make that a thing for event stuff. Even if they do at some point, I wouldn't expect it in the near future.