The three parts are technical knowledge, operating knowledge and knowledge of regulations. Operating and regulations are the same for all classes.
The technical part differs: class N draws its questions only from the N pool, class E from N and E, class A from all three pools. For the class A technical part you get 60 minutes instead of 45.
Parts you pass stay passed. If you fail one part, you only retake that one.
A calculator and the official aids are allowed. The formula sheet is provided in the exam, so you don't have to memorise formulas, just know which one you need when.