Alright, so now we've determined that the serebii calculator IS in fact correct, as I got the same perfect IVs as you did, however, the statement that that egg group is always 3 perfect IVs is now no longer true, as my Diancie proves:
https://www.dropbox.com/s/u0t5dfja05vkz6m/photo.JPG?dl=0
and that is the first Diancie I got that had ANY perfect IVs (this one only has two, but I kept it thinking that it might be the best one I get) however, at least we can rule out that the Serebii calculator was wrong because it's apparently not.