The reward will be given to the Top 3 of each Advanced Class of the entire Ranked population. NOT per server.
Presumably you are suggesting that the 3 highest rated players will get the reward. If so, this is stupid because rating from completely isolated player pools are incomparable. It should be obvious that this is the case but if you still need convincing here is a quick and dirty simulation I wrote to illustrate the issue:
See the end of jelo.py above for output from a sample run.
Depending on the makeup of the player pool, 2 players with the same "true rating" (Alice and Malice, true rating 2000) end up with significantly different actual ratings (1797 and 1484). These ratings are perfectly consistent relative to the ratings of their opponents (i.e. the difference between different player's actual ratings is very close to the diffence between their true ratings) but meaningless when compared across player pools.
In short, the more derps your population has, the higher rating you can achieve (assuming your skill remains constant).
It doesn't affect me personally because I'm a bad
and won't win the top tier reward regardless, I just enjoy pointing out the silliness of your proposed system.
OTOH, perhaps you have some other idea for comparing player performance across isolated player pools. In this case I look forward to hearing what you plan.