Class switching can only narrow down the results to 3 possibilities per drop, but the 3 possibilities (from 12 total) are fixed when you pick up the cursed item. I just work on the principle that if the drop I get first time is something I already have, it cannot ever be something different from the same colour class, so I'll swap jobs until it drops anything I dont have.
As I wrote, I think there's some sort of hidden hierarchy in place according to what gear you have already and from where the drop came.
The way dragonforging works might offer a clue in that the more you've upgraded a given piece of gear, the better the likelihood that it will be dragonforged when you kill a dragon-family monster.
I wouldn't get hung up too much on getting lvl3's purified though, in most cases a gold rarified lvl2 equivalent will outmatch them until you have it fully maxed out, which as they typically require Daimon or Death's rare-ass crafting item drops, are not quick and easy to get.