If your looking to fly in and out of major airports, Pittsburgh would probably make the most sense as Kennywood is right outside the city. The three parks you picked make the most sense based on what your looking for. You could add Dorney simply because it's relatively close to Hershey. Waldameer is a nice park too, but it seems more out of your way. The way I see it you could fly into Pittsburgh and fly out of Philly if you want to keep to the major airports.