As the holder of a Gr III Instructors Rating, you will be able to conduct (initial) Ab Initio training on the aircraft you are rated on. After a certain amount of instruction, you will be able to obtain your Gr II instructors rating. This will allow you to conduct more advanced training (Night Ratings, Instrument Ratings, PPL Skills Tests etc).
The requirements to obtain an Instructor's Rating:
A valid CPL or higher
A valid Instrument Rating
Pass the theoretical Exams
20 hours Instruction (Patter)
Flight test with a Designated Examiner