I made a topic about this a while ago on the Black 2 board, but the reading comprehension of the people replying left something to be desired. Anyway, there are a number of Pokemon who could easily have third type (Volcarona and Blaziken could very well have been part Flying, Charizard and Gyarados are natural fits as Dragon-types, White Kyurem and Black Kyurem were at one point almost expected to be part Fire and Electric respectively), but triple typings could quickly make things overly complicated, not to mention result in some defensively disastrous typings (A Fire/Bug/Flying type is 8 times weak to rocks for example).
A better way to shake things up would be to simply create abilities that give a 50% boost (the same as the boost from STAB) to moves of a particular type. It would make new moves viable for certain Pokemon, without causing all the complications that triple typings would bring, and it would certainly make sense for a number of Pokemon (Volcarona has six wings, yet doesn't get STAB on Hurricane? What a load of BS!).
I don't really see Blaziken as being a possible Flying type, though.
It's definitely a bird (remember Torchic?), and it can learn plenty of flying type moves, including Brave Bird. The only issue for Blazechicken would be that it already has a much better hidden ability, so it's not like it would actually benefit from such an addition.
Fair enough, but since bird type moves and Pokemon don't exist, it still makes sense to me that Blaziken should get a boost on the flying moves that it can learn, since they are generally clear "Bird-moves" (it's not like it can learn Fly anyway).
Regardless, it's not like it needs such an ability to begin with. Now Volcarona on the other hand..