Ok, I was actually curious about this. In the "past" of Platinum there used to be a plaque in the church that would tell you the tales of Dialga and Palkia. So curiously I went around to some homes in the game and there was a character that once read the plaque and remembered that it had to do with Dialga and Palkia and that there used to be a third Plaque that he didn't remember what it said. now being that you have Dialga, Palkia, and Giratina being the dragon trio's of the game, I'm going to take a long shot and say that it is Giratina's plaque.
Another theory of mine is it being related to Arceus. You know. Arceus being the "god" Pokemon and a cathedral? hmmm.
This made me then question the plague being Giratina's being that Giratina had a whole new dimension for itself. (Supposedly the "hell" of the game considering how dreary it looks maybe? I don't really know.)
My two guesses are the third plaque being for Giratina, the third 1/3 to dialga and plakia. Or it being for Arceus, being "god" and all.
Hope this helps.