I have never been told this and only recently learned it; I was surprised by what excellent IVs my legends had...how lucky I thought I was to catch a Regirock with 3 perfect ivs and a nearly perfect in Attack xD and a Premier Ball on the first try.... but that's irrelevant,
I suppose it's only fair they implemented this, but seriously how long have they done this?
They started giving the legendaries 3 perfect IVs in X and Y. There's also a chance that they increased the odds of getting more than 3 perfect IVs as well.
Fun fact: Special Pokemon such as the Cosplay Pikachu also have 3 perfect IVs.
Source: Experience and legendaries from gen 5 and below not having any perfect IVs