Only time will tell but at this point in time, it is rare since you can only get it from the limited time gen3 eggs on the server store!
It' impossible to tell since rares are only set just before the release. If your wondering because you want to trade for wailmer, just do it , someone will always buy it off you.
There is alot of demand for the pokemon Wailmer, so looking at this logically, it probably will be a rare pokemon in gen 3. But can't be too sure either
I would say Wailmer and Wailord has turned to somewhat a joke in the Pokémon Community, it shouldn't be so rare, I would say it would be a common grinded Pokémon in Gen 3 but anyways, only time can tell P.s you are my 200th Message! :tada: