Will Tesla's batteries be standard/inter-changable between different models? Or will 3 different battery sizes need to be stocked at each Tesla (swapping/charging)Station?
They wont be interchangeable.
AFAIK the Model S and X use the same base (the skateboard), so I'd imagine they could swap batteries. The Gen 3 is likely going to be an entirely new battery design since the car is smaller and hence can't accomodate such a large pack.