You meant dialga right? If your playing perfect soul the arceus event will not activate. But the conditions in finding the 3 sinnoh legendaries( I mean all of them) are:
Complete 28 unowns
Complete 16 gym badges
(that's what I did haha)
I'm trying to find them in perfect heart at early game without any gym badges and I can't seem to find them thats why I came up with the conditions above