I am missing 3 random artifacts and so far I have entered only the following maps: Daggerwound Islands, Ravenshore, Garrote Gorge, Alvar.
I got the bulk of the random artifacts from looting slain dragons. None of the maps above has respawned yet. I also cleared the Nagas dungeon and picked up one random artifact there.
I have been loading and saving before looting dragons dozens of times, but I still can't seem to find those last three random artifacts.