This question has been asked a lot; the fact is that no one knows what the likelihood of the iPad Air 3 getting any of the features of the iPad Pro, but I personally do not believe that it will get the ability to use the Pencil nor will it get stereo speakers like the iPP has. First, I don't believe that Apple will want to dilute the purpose of the iPP by making the Pencil a universal iOS accessory. Second, the Air really doesn't have the internal space necessary to make speakers like the ones in the iPP possible.
I imagine that the iPA3 will probably get a speed bump, Touch ID 2 sensor, and maybe Force Touch if Apple got the issues with that worked out. But the iPP will still have differentiating features, and I think that aside from its size the Pencil and the speaker quality are major ones.